[AMDGPU] divergence patterns for the BUILD_VECTOR i16, undef expansion.

BUILD_VECTOR of i16 and undef gets expanded to the COPY_TO_REGCLASS.
         The latter is further lowererd to the copy instructions.
	 We need to provide the correct register class for the uniform and divergent BUILD_VECTOR nodes
	 to avoid VGPR to SGPR copies.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D122068