[InstCombine] Fold frexp of select to select of frexp (#121227)

This patch implements an optimization to push select operations through
frexp when one of the select operands is a constant. When we encounter:
```
define float @src(float %x, i1 %bool) {
  %select = select i1 %bool, float 1.000000e+00, float %x
  %frexp = tail call { float, i32 } @llvm.frexp.f32.i32(float %select)
  %frexp.0 = extractvalue { float, i32 } %frexp, 0
  ret float %frexp.0
}
```
We transform it to:
```
define float @tgt(float %x, i1 %bool) {
  %frexp = tail call { float, i32 } @llvm.frexp.f32.i32(float %x)
  %frexp.0 = extractvalue { float, i32 } %frexp, 0
  %select = select i1 %bool, float 5.000000e-01, float %frexp.0
  ret float %select
}
```

Fixes #92542