[GlobalISel] Add a constant folding combine.

[aemerson]

Summary

These don't always get folded because when the instructions are created the constants are obscured by artifacts, or other reasons.

Use it AArch64 post-legal combiner too.

[paquette]

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

4267

This looks like it could use replaceInstWithConstant; does it need APInt? (Or should that helper be changed to use APInt?)

[aemerson]

Add a variant of replaceInstWithConstant taking APInt. It still seems convenient to have an int64_t version since that one builds the constant with the type of the DstOp, while we'd have to construct a correctly sized APInt in each call site if we replaced it.

[aemerson]

Forgot to remove old apply method.

[paquette]

LGTM

[foad]

These don't always get folded because when the instructions are created the constants are obscured by artifacts, or other reasons.

For the AMDGPU fshl/fshr tests, the reason is that the foldable instructions are created during legalization, and the AMDGPU legalizer doesn't use CSEMIRBuilder yet.

[foad]

In D106776#2910090, @foad wrote:

These don't always get folded because when the instructions are created the constants are obscured by artifacts, or other reasons.

For the AMDGPU fshl/fshr tests, the reason is that the foldable instructions are created during legalization, and the AMDGPU legalizer doesn't use CSEMIRBuilder yet.

Scratch that. The reason is that the constants are obscured by artifacts like G_ZEXT, which we don't constant fold (I previously tried that in D89392).