[llvm] [GlobalISel] Enhance iPTR type support in SDAG patterns (PR #111503)

[Evgenii Kudriashov via llvm-commits]

e-kud wrote:

> This is just a bug, the vector index type is separate from the pointer type

@arsenm if this is a bug it's already buried too deep.
```
/// Returns the type to be used for the index operand vector operations. By
/// default we assume it will have the same size as an address space 0
/// pointer.
virtual unsigned getVectorIdxWidth(const DataLayout &DL) const {
  return DL.getPointerSizeInBits(0);
}

/// Returns the type to be used for the index operand of:
/// ISD::INSERT_VECTOR_ELT, ISD::EXTRACT_VECTOR_ELT,
/// ISD::INSERT_SUBVECTOR, and ISD::EXTRACT_SUBVECTOR
MVT getVectorIdxTy(const DataLayout &DL) const {
  return MVT::getIntegerVT(getVectorIdxWidth(DL));
}

/// Returns the type to be used for the index operand of:
/// G_INSERT_VECTOR_ELT, G_EXTRACT_VECTOR_ELT,
/// G_INSERT_SUBVECTOR, and G_EXTRACT_SUBVECTOR
LLT getVectorIdxLLT(const DataLayout &DL) const {
  return LLT::scalar(getVectorIdxWidth(DL));
}
```

So each target that has vectors implemented in SDAG must overwrite default interface and whole SDAG implementation to enable vector patterns in GlobalISel. It looks like supporting `iPTR` as a HW dependent integer type could be more straightforward in terms of GlobalISel adoption. What do you think?

cc @aemerson @davemgreen @topperc 

https://github.com/llvm/llvm-project/pull/111503