[AArch64] Simplify MacroFusion

This patch assumes that the dependents to be scanned for the ExitSU are its
predecessors; otherwise, the successors of the instr are scanned.

Furthermore, sometimes the ExitSU was being fused twice, since it may be
fused once when scanning the successors from the beginning of the BB and
then again when scanning the predecessors of ExitSU.  Thus, when scanning
the successors of an instr, skip the ExitSU.

llvm-svn: 299974