AMDGPU: Select VOP3 form of sub

AMDGPU: Select VOP3 form of sub

The VOP3 form should always be the preferred selection form to be
shrunk later.

The r600 sub test needs to be split out because it asserts on the
arguments in the new test during the calling convention lowering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359899 91177308-0d34-0410-b5e6-96231b3b80d8