[AMDGPU] Move call clobbered return address registers s[30:31] to callee saved range

Currently the return address ABI registers s[30:31], which fall in the call
clobbered register range, are added a

[AMDGPU] Move call clobbered return address registers s[30:31] to callee saved range

Currently the return address ABI registers s[30:31], which fall in the call
clobbered register range, are added as a live-in on the function entry to
preserve its value when we have calls so that it gets saved and restored
around the calls.

But the DWARF unwind information (CFI) needs to track where the return address
resides in a frame and the above approach makes it difficult to track the
return address when the CFI information is emitted during the frame lowering,
due to the involvment of understanding the control flow.

This patch moves the return address ABI registers s[30:31] into callee saved
registers range and stops adding live-in for return address registers, so that
the CFI machinery will know where the return address resides when CSR
save/restore happen during the frame lowering.

And doing the above poses an issue that now the return instruction uses undefined
register `sgpr30_sgpr31`. This is resolved by hiding the return address register
use by the return instruction through the `SI_RETURN` pseudo instruction, which
doesn't take any input operands, until the `SI_RETURN` pseudo gets lowered to the
`S_SETPC_B64_return` during the `expandPostRAPseudo()`.

As an added benefit, this patch simplifies overall return instruction handling.

Note: The AMDGPU CFI changes are there only in the downstream code and another
version of this patch will be posted for review for the downstream code.

Reviewed By: arsenm, ronlieb

Differential Revision: https://reviews.llvm.org/D114652

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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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