[X86][AVX] Add support for shuffle decoding of vperm2f128/vperm2i128 with zero'd lanes

The vperm2f128/vperm2i128 shuffle mask decoding was not attempting to deal with shuffles that give zero lanes. This patch fixes this so that the assembly printer can provide shuffle comments.

As this decoder is also used in X86ISelLowering for shuffle combining, I've added an early-out to match existing behaviour. The hope is that we can add zero support in the future, this would allow other ops' decodes (e.g. insertps) to be combined as well.

Differential Revision: http://reviews.llvm.org/D10593

llvm-svn: 241516