[AMDGPU][GISel] Legalize V2S16 G_BUILD_VECTOR

Preparation patch for D134354 to make V2S16 G_BUILD_VECTOR legal.
Also removes RegBankInfo's scalarization of small BUILD_VECTORs,
replacing it with InstructionSelector logic instead.

This allows for V2S16 BUILD_VECTOR instructions to survive
all the way to ISel so we can select FMA/MAD_MIX instructions
in D134354.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D134433