Bugfixes for mask/swizzle handling

Closed Alyssa Rosenzweig requested to merge alyssa/mesa:ci/pre-simd-dce into master

In preparation for SIMD-aware dead code elimination, in turn in preparation for LCRA, in turn in preparation for 8/16/64-bit support, in turn in preparation for compute.

Oh my. I really am down a rabbit hole.

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