[X86] Move all the SSE legality checks out of FP_TO_INTHelper and up to LowerFP_TO_INT. NFCI

These checks aren't needed on the call to FP_TO_INTHelper from the type legalizer for splitting i64. We always want to use X87 FIST/FISTT to memory there.

Moving up the SSE checks will allow this routine to focus on what it cares about and makes its return semantics cleaner.

llvm-svn: 354161