[SystemZ] Support Compare and Traps

Support and generate Compare and Traps like CRT, CIT, etc.

Support Trap as legal DAG opcodes and generate "j .+2" for them by default.
Add support for Conditional Traps and use the If Converter to convert them into
the corresponding compare and trap opcodes.

Differential Revision: http://reviews.llvm.org/D21155

llvm-svn: 272419