[AVX512] Modify what indices we emit for the zero vector we use for zero...

[AVX512] Modify what indices we emit for the zero vector we use for zero extension of the result of a v2i1 or v4i1 masked compare. This way we emit something that the backend easily interprets as a concatenation rather than a true shuffle. This delivers slightly better codegen with the current backend capabilities.

llvm-svn: 274484