AMDGPU/SI: Consolidate FLAT patterns

Summary:
We had to sets of identical FLAT patterns one inside the
HasFlatAddressSpace predicate and one inside the useFlatForGloabl
predicate.  This patch merges these sets into a single pattern
under the isCIVI predicate.

The reason we can remove the predicates is that when MUBUF instructions
are legal, the instruction selector will prefer selecting those over
FLAT instructions because MUBUF patterns have a higher complexity score.
So, in this case having patterns for FLAT instructions will have no effect.

This change also simplifies the process for forcing global address space
loads to use FLAT instructions, since we no only have to disable the
MUBUF patterns instead of having to disable the MUBUF patterns and
enable the FLAT patterns.

Reviewers: arsenm, cfang

Subscribers: llvm-commits
llvm-svn: 256807