[ARM][AArch64][DAG] Reenable post-legalize store merge

Reenable post-legalize stores with constant merging computation and
corresponding test case.

 * Properly truncate store merge constants
 * Disable merging of truncated stores floating points
 * Ensure merges of constant stores into a single vector are
   constructed from legal elements.

Reviewers: eastig, efriedma

Reviewed By: eastig

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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319899 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed