[llvm] [PowerPC] Remove `UnsafeFPMath` uses (PR #154901)

[via llvm-commits]

paperchalice wrote:

Initially, I want to check `afn` for this operation, but I noticed the comment
```
// The nearbyint variants are not allowed to raise the inexact exception
// so we can only code-gen them with unsafe math.
```
and the doc string (sorry I couldn't find the doc about `xsrdpic` in IBM doc site)
```
// Rounding Instructions respecting current rounding mode
```
So I think the constrained intrinsic is more appropriate for this situation.
CC @andykaylor 

------

I also wonder why the vector fp types are not wrapped in `TM.Options.UnsafeFPMath`.


https://github.com/llvm/llvm-project/pull/154901