[llvm] [GlobalISel] An optimizing MIR builder (PR #132282)

[John Stuart via llvm-commits]

john-stuart2 wrote:

We have constant-folding in the CSEMIRbuilder, but it has no access to LegalizerInfo.
https://github.com/llvm/llvm-project/blob/0ec94983c4cf9183f0768d6e76b363e5ebc6b255/llvm/lib/CodeGen/GlobalISel/CSEMIRBuilder.cpp#L234
On which targets is it legal to build the G_BUILD_VECTOR underneath?

And I cannot you use CSEMIRBuilder without constant folding.

https://github.com/llvm/llvm-project/pull/132282