AVX-512: Fixed COPY_TO_REGCLASS for mask registers

AVX-512: Fixed COPY_TO_REGCLASS for mask registers

Copying one mask register to another under BW should be done with kmovq instruction, otherwise we can loose some bits.
Copying 8 bits under DQ may be done with kmovb.

Differential Revision: http://reviews.llvm.org/D14812

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253563 91177308-0d34-0410-b5e6-96231b3b80d8