[llvm] [GlobalISel] Use getCmpLibcallReturnType() for FCMP libcalls (NFC) (PR #226813)

[Kiroo via llvm-commits]

zlfn wrote:

Hi @davemgreen @arsenm, 
you reviewed the FCMP libcall lowering in #98452, so I'd appreciate it if you could take a look at this when you have time.

https://github.com/llvm/llvm-project/pull/226813