[X86] AVX512: Fix asm syntax for packed vcmp

The *_alt defs for vcmp are used by the InstParser (the asm string in the main
def is used by the InstPrinter) .  The former was accepting vector registers
as destination rather than mask registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211750 91177308-0d34-0410-b5e6-96231b3b80d8