AMDGPU: don't match vgpr loads for constant loads

Author: Dave Airlie <airlied@redhat.com>

In order to implement indirect sampler loads, we don't
want to match on a VGPR load but an SGPR one for constants,
as we cannot feed VGPRs to the sampler only SGPRs.

this should be applicable for llvm 3.7 as well.

llvm-svn: 243294