[X86] Add the bizarro movsww and movzww mnemonics for the disassembler.

The destination size of the movzx/movsx instruction is controlled by the normal operand size mechanisms. Only the input type is fixed.

This means that a 0x66 prefix on the encoding for zext/sext 16->32 should really produce a 16->16 instruction. Functionally this is equivalent to a GR16->GR16 move since bits 16 and above will be preserved. So nothing is actually extended.

llvm-svn: 330078