[X86] Refactor the FP_TO_INTHelper interface. NFCI

-Pull the final stack load creation from the two callers into the helper.
-Return a single SDValue instead of a std::pair.
-Remove the Replace flag which isn't really needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353920 91177308-0d34-0410-b5e6-96231b3b80d8