[SystemZ] Rework IPM sequence generation

The SystemZ back-end uses a sequence of IPM followed by arithmetic
operations to implement the SETCC primitive.  This is currently done
early during SelectionDAG.  This patch moves generating those sequences
to much later in SelectionDAG (during PreprocessISelDAG).

This doesn't change much in generated code by itself, but it allows
further enhancements that will be checked-in as follow-on commits.

llvm-svn: 322987