[llvm] [SDAG][X86] Remove hack needed to avoid missing x87 FPU stack pops (PR #128055)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Do we need a regression for X86, or it's covered by some existing test?

It's already covered by tests like https://github.com/llvm/llvm-project/blob/main/llvm/test/CodeGen/X86/llvm.frexp.ll (that it does not change in this PR means its working). 

https://github.com/llvm/llvm-project/pull/128055