[SystemZ] Register compare-and-branch support

This patch adds support for the CRJ and CGRJ instructions.  Support for
the immediate forms will be a separate patch.

The architecture has a large number of comparison instructions.  I think
it's generally better to concentrate on using the "best" comparison
instruction first and foremost, then only use something like CRJ if
CR really was the natual choice of comparison instruction.  The patch
therefore opportunistically converts separate CR and BRC instructions
into a single CRJ while emitting instructions in ISelLowering.