add TableGen support to create relationship maps between instructions

Relationship maps are represented as InstrMapping records which are parsed by
TableGen and the information is used to construct mapping tables to represent
appropriate relations between instructions. These tables are emitted into
XXXGenInstrInfo.inc file along with the functions to query them.

Patch by Jyotsna Verma <jverma@codeaurora.org>.

llvm-svn: 166685