Second stab at target-dependent lowering of everyone's favorite nodes: [SU]ADDO

- LowerXADDO lowers [SU]ADDO into an ADD with an implicit EFLAGS define. The
  EFLAGS are fed into a SETCC node which has the conditional COND_O or COND_C,
  depending on the type of ADDO requested.

- LowerBRCOND now recognizes if it's coming from a SETCC node with COND_O or
  COND_C set.

llvm-svn: 60388