[DSE] Merge stores when the later store only writes to memory locations the...

[DSE] Merge stores when the later store only writes to memory locations the early store also wrote to.

Summary:
This fixes PR31777.

If both stores' values are ConstantInt, we merge the two stores
(shifting the smaller store appropriately) and replace the earlier (and
larger) store with an updated constant.

In the future we should also support vectors of integers. And maybe
float/double if we can.

Reviewers: hfinkel, junbuml, jfb, RKSimon, bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30703

llvm-svn: 310055