[X86] Remove 64/128/256 from MMX/SSE/AVX instruction names for overall consistency. NFC

MMX instrutions all start with MMX_ so the 64 isn't needed for disambigutation.
SSE/AVX1 instructions are assumed 128-bit so we don't need to say 128.
AVX2 instructions should use a Y to indicate 256-bits.

llvm-svn: 323402