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

[John Stuart via llvm-commits]

john-stuart2 wrote:

CSEMIRBuilder constant folds G_SITOFP and G_UITOFP. The parameters switch between integers and floats, but more importantly the bit width may change. For all cast operations, the input and output differ. I would prefer to the query the legalizer before folding.

mul(undef) combines into 0.
```
 if ((isUndef(SrcOps[1].getReg()) || isUndef(SrcOps[0].getReg())) &&
      isConstantLegalOrBeforeLegalizer(DstTy))
    return buildConstant(DstTy, 0);
```
My PR checks whether it is legal to write 0 into the destination.

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