AMDGPU: Mark more instructions as rematerializable

D106023 excluded 16-bit instructions from rematerialization, with the
justification that we can't rematerialize instructions that preserve
the high bits (plus the instructions which do are a confusing mess
between different subtargets). This doesn't make sense to me as a
problem since cases where we would rely on the high bit behavior would
still need to be represented as a register value constraint with a
tied operand. It's not a hidden side effect and should still be
rematerializable.