[X86] Strengthen type constraints on some specialized X86 ISD opcodes that...

[X86] Strengthen type constraints on some specialized X86 ISD opcodes that don't have any flexibility. NFC

These particular instructions only operate on 128-bit vectors and have no wider equivalents. And the
element size is always known.

One could argue that MOVSS/MOVSD could be merged, but that's probably disruptive to code in
X86ISelLowering and probably low value.

llvm-svn: 360815