[X86] Teach X86DAGToDAGISel::shrinkAndImmediate to preserve upper 32 zeroes of a 64 bit mask.

If the upper 32 bits of a 64 bit mask are all zeros, we have special isel patterns to use a 32-bit and instead of a 64-bit and by relying on the impliciting zeroing of 32 bit ops.

This patch teachs shrinkAndImmediate not to break that optimization.

Differential Revision: https://reviews.llvm.org/D42899

llvm-svn: 324249