[X86] Don't zero the upper bits of the k-register before extracting a single bit...

[X86] Don't zero the upper bits of the k-register before extracting a single bit from a vXi1.

This doesn't match the semantics of the extract_vector_elt operation. Nothing downstream knows the bits were zeroed so they still get masked or sign extended after the extrat anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320723 91177308-0d34-0410-b5e6-96231b3b80d8