nir: Remove Some Masking Before Unpacking Instructions

Removes the masking in unpack_{half_2x16,32_2x16,64_2x32}_split_{x,y}(a & mask_appropriate_for_unpack).

I didn't want to create huge commit messages, so here are some shader-db/vkpipeline-db results in Skylake: https://gitlab.freedesktop.org/snippets/633