Teach BaseIndexOffset::match to identify base pointers in loops.

Teach BaseIndexOffset::match to identify base pointers in loops.

The small utility function that pattern matches Base + Index +
Offset patterns for loads and stores fails to recognize the base
pointer for loads/stores from/into an array at offset 0 inside a
loop. As a result DAGCombiner::MergeConsecutiveStores was not able
to merge all stores.

This commit fixes the issue by adding an additional pattern match
and also a test case.

Reviewer: Nadav

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188936 91177308-0d34-0410-b5e6-96231b3b80d8