[AMDGPU] fix copies between 32 and 16 bit

This a hack to fix illegal 32 to 16 bit copies.
The problem is when we make 16 bit subregs legal it creates
a huge amount of failures which can only be resolved at once
without a temporary hack like this.

The next step is to change operands, instruction definitions
and patterns until this hack is not needed.

Differential Revision: https://reviews.llvm.org/D79119