Changes

[gicombiner] Add parse failure tests for defs/match

AMDGPU: Fix the broken dominator tree when creating waterfall loop for
resource descriptor
Summary:
 In loadSRsrcFromVGPR, if MBB is the same as Succ, Remiander is not the
immediate dominator of Succ.
Reviewer:
 arsenm
Differential Revision:
 https://reviews.llvm.org/D69358

AMDGPU/GlobalISel: Handle flat/global G_ATOMIC_CMPXCHG
Custom lower this to a target instruction with the merge operands. I 
think it might be better to directly select this and emit a 
REG_SEQUENCE, but this would be more work since it would require 
splitting the tablegen patterns for these cases from the other atomics.