[X86] Remove isel patterns for mask vpcmpgt/vpcmpeq. Switch vpcmp to these...

[X86] Remove isel patterns for mask vpcmpgt/vpcmpeq. Switch vpcmp to these based on the immediate in MCInstLower

The immediate form of VPCMP can represent these completely. The
vpcmpgt/eq are just shorter encodings.

This patch removes the isel patterns and just swaps the opcodes
and removes the immediate in MCInstLower. This matches where we do
some other encodings tricks.

Removes over 10K bytes from the isel table.

Differential Revision: https://reviews.llvm.org/D68446

llvm-svn: 373766