AVX512 : Change v8i1 bitconvert GR8 pattern, remove unnecessary movzbl instruction.

code example , previous implementation.
    movzbl  %dil, %eax
    kmovw  %eax, %k0
  new code
    kmovw  %edi, %k0

Differential Revision: http://reviews.llvm.org/D16287

llvm-svn: 258045