[DAG] Defer Pre/Post IndexStore merge to after mergestore. NFCI.

In preparation for doing storemerge post-legalization, reorder
visitSTORE passes to move pre/post-index combining after store
merge. Reordered passes other than store merge are unaffected.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305473 91177308-0d34-0410-b5e6-96231b3b80d8
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