[X86] Remove isel checks for immediate size on floating point compare and xop...

[X86] Remove isel checks for immediate size on floating point compare and xop compare instructions. NFCI

If these checks fail we end up not selecting an instruction at all. So we are already relying on the immediate being checked upstream of isel. So doing the check in isel is just bloat to the isel table. Interestingly, we didn't check on the AVX512 version of the instructions anyway.

llvm-svn: 313724