[X86] Derive ssmem and sdmem from X86MemOperand. NFCI

This changes the operand type from v4f32/v2f64 to iPTR which seems more correct. But that doesn't seem to do anything other than change the comments in X86GenDAGISel.inc. Probably because we use a ComplexPattern to do the matching so there's no autogenerated code to change.

llvm-svn: 357959