[x86][inline-asm][avx512] allow swapping of '{k<num>}' & '{z}' marks

Committing on behalf of Coby Tayree: After check-all and LGTM

Desc:

AVX512 allows dest operand to be followed by an op-mask register specifier ('{k<num>}', which in turn may be followed by a merging/zeroing specifier ('{z}')
 Currently, the following forms are allowed:
 {k<num>}
 {k<num>}{z}

This patch allows the following forms:
 {z}{k<num>}

and ignores the next form:
 {z}

Justification would be quite simple - GCC

Differential Revision: http://reviews.llvm.org/D25013

llvm-svn: 284479