[DAGCombiner] Bugfix for lost chain depenedency.

When MergeConsecutiveStores() combines two loads and two stores into
wider loads and stores, the chain users of both of the original loads
must be transfered to the new load, because it may be that a chain
user only depends on one of the loads.

New test case: test/CodeGen/SystemZ/dag-combine-01.ll

Reviewed by James Y Knight.

Bugzilla: https://llvm.org/bugs/show_bug.cgi?id=25310#c6
llvm-svn: 253779