[llvm] [LangRef] Clarify nsz semantics (PR #180906)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

Here's one test that breaks with this semantics:
```llvm
; llvm/test/Transforms/InstCombine/intrinsic-select.ll

define double @test_fabs_select_fmf2(i1 %cond, double %a) {
  %sel1 = select i1 %cond, double 0.000000, double %a
  %fabs = fabs nnan ninf nsz double %sel1
  ret double %fabs
}
=>
define double @test_fabs_select_fmf2(i1 %cond, double %a) {
  %#1 = fabs double %a
  %fabs = select nnan ninf nsz i1 %cond, double 0.000000, double %#1
  ret double %fabs
}
Transformation doesn't verify! (unsound)
ERROR: Target's return value is more undefined

Example:
i1 %cond = #x0 (0)
double %a = #x0000000000000000 (+0.0)

Source:
double %sel1 = #x0000000000000000 (+0.0)
double %fabs = #x0000000000000000 (+0.0)

Target:
double %#1 = #x0000000000000000 (+0.0)
double %fabs = #x8000000000000000 (-0.0)
Source value: #x0000000000000000 (+0.0)
Target value: #x8000000000000000 (-0.0)
```

https://github.com/llvm/llvm-project/pull/180906