AMDGPU/GlobalISel: Look through copies in getPtrBaseWithConstantOffset

We may have an SGPR->VGPR copy if a totally uniform pointer
calculation is used for a VGPR pointer operand.

Also hack around a bug in MUBUF matching which would incorrectly use
MUBUF for global when flat was requested. This should really be a
predicate on the parent pattern, but the DAG always checked this
manually inside the complex pattern.