[X86] Rename vy512mem->vy512xmem and vz256xmem->vz256mem.

The index size is represented by the letter after the 'v'. The number represents the memory size. If an 'x' appears after the number its means the index register can be from VR128X/VR256X instead of VR128/VR256.

As vy512mem uses a VR256X index it should have an x.
And vz256mem uses a VR512 index so it shouldn't have an x.

I admit these names kind of suck and are confusing.

llvm-svn: 334120