Set "needFallThroughBranch" flag during block split.

Bonus changes:
* Add a undefined Thumb instruction between the last code block and the
  beginning of PC reconstruction cell to capture such codegen problem on
  the spot.
* Fix a loop formation problem to exclude nested loops.

Bug: 4320840
Change-Id: I49d3fbba0073d8c2d4a0b241258239cb952c6bdd
5 files changed