Extend a trace with a backward branch into a loop.

Extend a trace with a backward branch into a loop.

When seeing a trace that ends with a backward branch, exhaust all code
blocks reachable from that trace and try to identify if there exists a
non-nested loop. If the derived loop is found to be too complex or only
acyclic code is seen, revert to the original compilation mechanism to
translate a simple trace.

This CL uses the whole-method parser/dataflow analysis framework to
identify such loops. No optimization/codegen are performed yet.

Bug: 4086718

Change-Id: I19ed3ee53ea1cbda33940c533de8e9220e647156