[ARM][DAG] Reenable post-legalize store merge

Summary: Reenable post-legalize stores with constant merging computation and cofrresponding test case.

Reviewers: eastig, efriedma

Subscribers: aemerson, javed.absar, kristof.beyls, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D40701

llvm-svn: 319547