[clang] [clang][CodeGen] Remove "unsafe-fp-math" attribute support (PR #162779)

[via cfe-commits]

paperchalice wrote:

> I'm very happy to see progress being made on transitioning away from the function attributes for fast-math control. That said, it looks like there are still some things in the PowerPC ISel code that were using this attribute (by way of the UnsafeFPMath setting in TargetOptions).
> 
> Do you have plans to remove the backend handling of this attribute?

PowerPC part is in #15490, but it needs fast-math flags from uitofp, which is impossible in current implementation, because of nneg flag.

https://github.com/llvm/llvm-project/pull/162779