[mlir] Refactor ElementsAttr's value access API

There are several aspects of the API that either aren't easy to use, or are
deceptively easy to do the wrong thing. The main change of this commit
is

[mlir] Refactor ElementsAttr's value access API

There are several aspects of the API that either aren't easy to use, or are
deceptively easy to do the wrong thing. The main change of this commit
is to remove all of the `getValue<T>`/`getFlatValue<T>` from ElementsAttr
and instead provide operator[] methods on the ranges returned by
`getValues<T>`. This provides a much more convenient API for the value
ranges. It also removes the easy-to-be-inefficient nature of
getValue/getFlatValue, which under the hood would construct a new range for
the type `T`. Constructing a range is not necessarily cheap in all cases, and
could lead to very poor performance if used within a loop; i.e. if you were to
naively write something like:

```
DenseElementsAttr attr = ...;
for (int i = 0; i < size; ++i) {
// We are internally rebuilding the APFloat value range on each iteration!!
APFloat it = attr.getFlatValue<APFloat>(i);
}
```

Differential Revision: https://reviews.llvm.org/D113229

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[mlir][RFC] Refactor layout representation in MemRefType

The change is based on the proposal from the following discussion:
https://llvm.discourse.group/t/rfc-memreftype-affine-maps-list-vs-single-i

[mlir][RFC] Refactor layout representation in MemRefType

The change is based on the proposal from the following discussion:
https://llvm.discourse.group/t/rfc-memreftype-affine-maps-list-vs-single-item/3968

* Introduce `MemRefLayoutAttr` interface to get `AffineMap` from an `Attribute`
(`AffineMapAttr` implements this interface).
* Store layout as a single generic `MemRefLayoutAttr`.

This change removes the affine map composition feature and related API.
Actually, while the `MemRefType` itself supported it, almost none of the upstream
can work with more than 1 affine map in `MemRefType`.

The introduced `MemRefLayoutAttr` allows to re-implement this feature
in a more stable way - via separate attribute class.

Also the interface allows to use different layout representations rather than affine maps.
For example, the described "stride + offset" form, which is currently supported in ASM parser only,
can now be expressed as separate attribute.

Reviewed By: ftynse, bondhugula

Differential Revision: https://reviews.llvm.org/D111553

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[mlir] Refactor ElementsAttr into an AttrInterface

This revision refactors ElementsAttr into an Attribute Interface.
This enables a common interface with which to interact with
element attributes, w

[mlir] Refactor ElementsAttr into an AttrInterface

This revision refactors ElementsAttr into an Attribute Interface.
This enables a common interface with which to interact with
element attributes, without needing to modify the builtin
dialect. It also removes a majority (if not all?) of the need for
the current OpaqueElementsAttr, which was originally intended as
a way to opaquely represent data that was not representable by
the other builtin constructs.

The new ElementsAttr interface not only allows for users to
natively represent their data in the way that best suits them,
it also allows for efficient opaque access and iteration of the
underlying data. Attributes using the ElementsAttr interface
can directly expose support for interacting with the held
elements using any C++ data type they claim to support. For
example, DenseIntOrFpElementsAttr supports iteration using
various native C++ integer/float data types, as well as
APInt/APFloat, and more. ElementsAttr instances that refer to
DenseIntOrFpElementsAttr can use all of these data types for
iteration:

```c++
DenseIntOrFpElementsAttr intElementsAttr = ...;

ElementsAttr attr = intElementsAttr;
for (uint64_t value : attr.getValues<uint64_t>())
...;
for (APInt value : attr.getValues<APInt>())
...;
for (IntegerAttr value : attr.getValues<IntegerAttr>())
...;
```

ElementsAttr also supports failable range/iterator access,
allowing for selective code paths depending on data type
support:

```c++
ElementsAttr attr = ...;
if (auto range = attr.tryGetValues<uint64_t>()) {
for (uint64_t value : *range)
...;
}
```

Differential Revision: https://reviews.llvm.org/D109190

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