Special case common branch patterns in breakLoopBackedge (try 2)

Changes since aec08e:
* Adjust placement of a closing brace so that the general case actually runs.  Turns out we had *no* coverage of the switch case.  I added one in eae90fdc.
* Drop .llvm.loop.* metadata from the new branch as there is no longer a loop to annotate.

Original commit message:

This special cases an unconditional latch and a conditional branch latch exit to improve codegen and test readability. I am hoping to reuse this function in the runtime unroll code, but without this change, the test diffs are far too complex to assess.