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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354161 91177308-0d34-0410-b5e6-96231b3b80d8