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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

Can you pre-commit this test: https://alive2.llvm.org/ce/z/XNQvu5
```
define half @src(half %394) {
entry:
  %395 = fpext half %394 to double
   %396 = fcmp olt double %395, 0.000000e+00
   %397 = select i1 %396, double 0.000000e+00, double %395
   %398 = fptrunc double %397 to half
  ret half %398
}

define half @tgt(half %394) {
entry:
   %396 = fcmp olt half %394, 0.000000e+00
   %397 = select i1 %396, half 0.000000e+00, half %394
  ret half %397
}
```


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