[AMDGPU] Turn off MergeConsecutiveStores() before Instruction Selection for...

[AMDGPU] Turn off MergeConsecutiveStores() before Instruction Selection for AMDGPU. Commit dbbb6c5fc3642987430866dffdf710df4f616ac7 turned on MergeConsecutiveStores() before Instruction Selection for all targets. Enough AMDGPU compiles go into an infinite loop ( MergeConsecutiveStores() merges two stores; LegalizeStoreOps() un-merges; MergeConsecutiveStores() re-merges, etc. ) to warrant turning it off until the issues can be addressed.

Differential Revision: https://reviews.llvm.org/D41377

llvm-svn: 321100