[X86][AVX512] Change mask ops on vpermi2var tests to not use zeroinitializer.

This is necessary as I'm wanting to remove the 'Constant Pool' shuffle decoding from getTargetShuffleMask - but using getTargetShuffleMaskIndices allows the shuffle combiner to realize that these calls are really broadcasts.....

As with a lot of the X86ISD::VPERMV3 code this causes some vperm2i/vperm2t shuffles to flip depending on optimal commutation.

llvm-svn: 346032