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

[John Stuart via llvm-commits]

john-stuart2 wrote:

Should we remove them for compile time?
```
grep isLegalOrBeforeLegalizer llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp | wc -l
43
```
We have Legalizers for each target that precisely state what they consider legal. Sapphire Rapids and Thumb are different. I am confused about the it just works mentality.

What are the issues? Fail to select?

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