[DAGCombine] Fix a bug in MergeConsecutiveStores.

The bug manifests when there are two loads and two stores chained as follows in
a DAG,

(ld v3f32) -> (st f32) -> (ld v3f32) -> (st f32)

and the stores' values are extracted from the preceding vector loads.

MergeConsecutiveStores would replace the first store in the chain with the
merged vector store, which would create a cycle between the merged store node
and the last load node that appears in the chain.

This commits fixes the bug by replacing the last store in the chain instead.

rdar://problem/20275084

Differential Revision: http://reviews.llvm.org/D8849

llvm-svn: 234430