[X86] Allow KORTEST instruction to be used for testing if a mask is all ones

The KTEST instruction sets the C flag if the result of anding both operands together is all 1s. We can use this to lower (icmp eq/ne (bitcast (vXi1 X), -1)

Differential Revision: https://reviews.llvm.org/D42772

llvm-svn: 324577