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

[John Stuart via llvm-commits]

john-stuart2 wrote:

>From AArch64:
```
 getActionDefinitionsBuilder({G_SITOFP, G_UITOFP})
      .legalFor({{s32, s32},
                 {s64, s32},
                 {s32, s64},
                 {s64, s64},
                 {v2s32, v2s32},
                 {v4s32, v4s32},
                 {v2s64, v2s64}})
      .legalFor(HasFP16,
                {{s16, s32}, {s16, s64}, {v4s16, v4s16}, {v8s16, v8s16}})
...
```
Even under HasFP16, f16 constants are not legal. Constant folding G_SITOFP and G_UITOFP into f16 (or vectors of f16) is not legal.

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