[llvm] [InstCombine] Remove redundant fptrunc of select fold (NFCI) (PR #117182)

[Nikita Popov via llvm-commits]

nikic wrote:

A complicating factor is that we perform this canonicalization:
```llvm
define float @t4(double %a) {
; CHECK-LABEL: @t4(
; CHECK-NEXT:    [[DOTINV:%.*]] = fcmp oge double [[A:%.*]], 5.000000e+00
; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[DOTINV]], double 5.000000e+00, double [[A]]
; CHECK-NEXT:    [[TMP2:%.*]] = fptrunc double [[TMP1]] to float
; CHECK-NEXT:    ret float [[TMP2]]
;
  %1 = fcmp ult double %a, 5.0
  %2 = fptrunc double %a to float
  %3 = select i1 %1, float %2, float 5.0
  ret float %3
}
```

So just special casing fptrunc also doesn't work, because we'd infinite loop with this transform. So I'm inclined to just leave this alone...

https://github.com/llvm/llvm-project/pull/117182