[AMDGPU] Enable v4f16 and above for v_pk_fma instructions

Summary:
If isel is presented with <2 x half> vectors then it will correctly select
v_pk_fma style instructions.
If isel is presented with e.g. <4 x half> vectors it will scalarize, unlike for
other instruction types (such as fadd, fmul etc.)

Added extra support to enable this. Updated one of the tests to include a test
for this (as well as extending the test to GFX9)

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, tpr, t-tye, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65325

Change-Id: I50a4577a3f8223fb53992af3b7d26121f65b71ee
llvm-svn: 367206