[SelectionDAG] set dereferenceable flag in MergeConsecutiveStores to fix assetion failure

When SelectionDAG merges consecutive stores and loads in MergeConsecutiveStores, it does not set dereferenceable flag for a created load instruction. This results in an assertion failure if SelectionDAG commonizes this load instruction with other load instructions, as well as it may miss optimization opportunities.

This patch sat dereferenceable flag for the newly created load instruction if all the load instructions to be merged are dereferenceable.

Differential Revision: https://reviews.llvm.org/D34679

llvm-svn: 306404