[AMDGPU] Prevent too large store merges in AMDGPU Subtargets. NFCI.

Various address spaces on the SI and R600 subtargets have stricter
limits on memory access size that other address spaces. Use
canMergeStoresTo predicate to prevent the DAGCombiner from creating
these stores as they will be split up during legalization.

llvm-svn: 303767