[X86] Make masked pcmpeq commutable during isel so we can fold loads in other operand...

[X86] Make masked pcmpeq commutable during isel so we can fold loads in other operand to the shorter encoding.

Previously we used the immediate encoding if the load was in operand 0 and the short encoding if the load was in operand 1.

This added an insane number of bytes to the size of the isel table. I'm wondering if we should always use the immediate form during isel and change to the short form during emission. This would remove the need to pattern match every combination for both the immediate form and the short form during isel. We could do the same with vpcmpgt

llvm-svn: 325456