swift234

[g-arc-opts] Ignore trap BB during Sequence Dataflow.

These trap BBs are pattern matched very strictly, i.e. only BBs of the
following form are matched:

bbN:
  %_ = builtin_function_ref "int_trap"
  apply %_
  unreachable

Such BBs can be ignored safely for ARC purposes. This allows us to
handle -O3 code in the ARC optimizer significantly more effectively by
enabling us to handle many more types of check failures.

For instance we can now remove retains, releases in the following code:

bb0:
  strong_retain %0
  cond_br _, bb1, bb2

bb1:
  strong_release %0
  ...

bb2:
  %_ = builtin_function_ref "int_trap"
  apply %_
  unreachable

rdar://17212610

Swift SVN r18817