AMDGPU: Start selecting v_mad_mixlo_f16

Also add some tests that should be able to use v_mad_mixhi_f16,
but do not yet. This is trickier because we don't really model
the partial update of the register done by 16-bit instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313806 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed