[AMDGPU] Add v3f16/v3i16 support to SDag

Fix lowering and instruction selection for v3x16 types
and enable InstCombine to emit them.

This patch only implements it for the selection dag.
GlobalISel tests in GlobalISel/llvm.amdgcn.image.load.1d.d16.ll and
GlobalISel/llvm.amdgcn.image.store.2d.d16.ll still don't work.

Differential Revision: https://reviews.llvm.org/D84420