[BOLT] Fixes for new profile

Summary:
Do a better job of recording fall-through branches in new profile mode
(-prof-compat-mode=0). For this we need to record offsets for all
instructions that are last in the containing basic block.

Change the way we convert conditional tail calls. Now we never reverse
the condition. This is required for better profile matching.
The original approach of preserving the direction was controversial
to start with.

Add "-infer-fall-throughs" option (on by default) to allow disabling
inference of fall-through edge counts.

(cherry picked from FBD6994293)