[SystemZ] Bugfix in isFusableLoadOpStorePattern()

This function is responsible for checking the legality of fusing an instance
of load -> op -> store into a single operation. In the SystemZ backend the
check was incomplete and a test case emerged with a cycle in the instruction
selection DAG as a result.

Instead of using the NodeIds to determine node relationships,
hasPredecessorHelper() now is used just like in the X86 backend. This handled
the failing tests and as well gave a few additional transformations on
benchmarks.

The SystemZ isFusableLoadOpStorePattern() is now a very near copy of the X86
function, and it seems this could be made a utility function in common code
instead.

Review: Ulrich Weigand
https://reviews.llvm.org/D60255

llvm-svn: 357688