[SystemZ] Bugfix in adjustSubwordCmp.

Csmith generated a program where a store after load to the same address did
not get chained after the new load created during DAG legalizing, and so
performed an illegal overwrite of the expected value.

When the new zero-extending load is created, the chain users of the original
load must be updated, which was not done previously.

A similar case was also found and handled in lowerBITCAST.

Review: Ulrich Weigand
https://reviews.llvm.org/D40542

llvm-svn: 319409