AMDGPU: Fix high 16-bit optimization on gfx9

We can do this optimization in the majority of cases, but we currently
don't have a way to do it. We do not track/model which instructions
have which behavior, the control bit to change the high bit behavior,
or making use of preserved bits at all. This is a bit fuzzy since we
don't know precisely how the source instruction will be lowered, but
that only really matters in one case (for fma_mixlo).

We do need to fixup some of these cases after selection, but the
pattern helps eliminate many of these zexts.

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