[DAG] Do MergeConsecutiveStores again before Instruction Selection

Summary:

Now that store-merge is only generates type-safe stores, do a second
pass just before instruction selection to allow lowered intrinsics to
be merged as well.

Reviewers: jyknight, hfinkel, RKSimon, efriedma, rnk, jmolloy

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D33675

llvm-svn: 319036