[SelectionDAG] set dereferenceable flag when expanding memcpy/memmove

When SelectionDAG expands memcpy (or memmove) call into a sequence of load and store instructions, it disregards dereferenceable flag even the source pointer is known to be dereferenceable.
This results in an assertion failure if SelectionDAG commonizes a load instruction generated for memcpy with another load instruction for the source pointer.
This patch makes SelectionDAG to set the dereferenceable flag for the load instructions properly to avoid the assertion failure.

Differential Revision: https://reviews.llvm.org/D34467

llvm-svn: 306209