[UpdateTestChecks] Auto-generate stub bodies for unused prefixes

This is scoped to autogenerated tests.

The goal is to support having each RUN line specify a list of
check-prefixes where one can sp

[UpdateTestChecks] Auto-generate stub bodies for unused prefixes

This is scoped to autogenerated tests.

The goal is to support having each RUN line specify a list of
check-prefixes where one can specify potentially redundant prefixes. For example,
for X86, if one specified prefixes for both AVX1 and AVX2, and the codegen happened to
match today, one of the prefixes would be used and the onther one not.
If the unused prefix were dropped, and later, codegen differences were
introduced, one would have to go figure out where to add what prefix
(paraphrasing
https://lists.llvm.org/pipermail/llvm-dev/2021-February/148326.html)

To avoid getting errors due to unused prefixes, whole directories can be
opted out (as discussed on that thread), but that means that tests that
aren't autogenerated in such directories could have undetected unused
prefix bugs.

This patch proposes an alternative that both avoids the above, dir-level
optout, and supports the main autogen scenario discussed first. The autogen
tool appends at the end of the test file the list of unused prefixes,
together with a note explaining that is the case. Each prefix is set up
to always pass.

This way, unexpected unused prefixes are easily discoverable, and
expected cases "just work".

Differential Revision: https://reviews.llvm.org/D124306

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[update_llc_test_checks] Use FileCheck captures for MCInst/MCReg output

To avoid test churn when backends add/rename new instructions/registers,
it makes sense to use FileCheck captures for the exac

[update_llc_test_checks] Use FileCheck captures for MCInst/MCReg output

To avoid test churn when backends add/rename new instructions/registers,
it makes sense to use FileCheck captures for the exact MCInst/Reg number.
This is motivated by D125307, where I use --asm-show-inst to differentiate
the output for multiple instructions with the same mnemonic.

This does not quite fix the churn issue yet: While files with the generated
checks will be immune to the numbers changing, the update script test
still suffers from this problem since the number is encoded in the
FileCheck variable name. I plan to address this in a follow-up patch.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D125307

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[UpdateTestChecks] Add NVPTX support in update_llc_test_checks.py

This patch makes possible generating NVPTX assembly check lines with
update_llc_test_checks.py utility.

Differential Revision: http

[UpdateTestChecks] Add NVPTX support in update_llc_test_checks.py

This patch makes possible generating NVPTX assembly check lines with
update_llc_test_checks.py utility.

Differential Revision: https://reviews.llvm.org/D122986

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[UpdateLLCTestChecks] Add support for isel debug output in update_llc_test_checks.py

Add a check on run lines to pick up isel options in llc commands and allow
generating check lines of isel final o

[UpdateLLCTestChecks] Add support for isel debug output in update_llc_test_checks.py

Add a check on run lines to pick up isel options in llc commands and allow
generating check lines of isel final output other than assembly. If llc command
line contains -debug-only=isel, update_llc_test_checks.py will try to scrub isel
output, otherwise, the script will fall back on default behaviour, which is try to
scrub assembly output instead.

The motivation of this change is to allow usage of update_llc_test_checks.py to
autogenerate checks of instruction selection results. In this way, we can detect
errors at an earlier stage before the compilation goes all the way to assembly.
It is an example of having some transparency for the stages between IR and
assembly. These generated tests are almost like "unit tests" of isel stage.

This patch only implements the initial change to differentiate isel output from
assembly output for Lanai. Other targets will not be supported for isel check
generation at the moment. Although adding support for it will only require
implementing the function regex and scrubber for corresponding targets.

The Lanai implementation was chosen mainly for the simplicity of demonstrating
the difference between isel checks and asm checks.

This patch also do not include the implementation of function prefix, which is
required for the generated isel checks to pass. I will put up a follow up revision
for the function prefix change to complete isel support.

Reviewed By: Flakebi

Differential Revision: https://reviews.llvm.org/D119368

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