This is part2 of https://reviews.llvm.org/D35839 and this change is related to AVX512.
VEX-encoded vxorps %xmm0, %xmm0, %xmm0 does the same thing as EVEX-encoded vxorps %zmm0, %zmm0, %zmm0, zeroing the full-width vector and breaking dependencies on the old value of the architectural register.
The VEX version is one byte shorter than the EVEX version.