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

[via llvm-commits]

RolandF77 wrote:

> @RolandF77 Do you have any opinions on this?

xsrdpic checks round mode, but may cause an inexact exception. There are other instructions that use a static rounding mode and don't raise an exception. So maybe we can use xsrdpiz if -fno-rounding-math?

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