AMDGPU: Try to eliminate clearing of high bits of 16-bit instructions

AMDGPU: Try to eliminate clearing of high bits of 16-bit instructions

These used to consistently be zeroed pre-gfx9, but gfx9 made the
situation complicated since now some still do and some don't. This
also manages to pick up a few cases that the pattern fails to optimize
away.

We handle some cases with instruction patterns, but some get
through. In particular this improves the integer cases.