prove used a complex logic when trying to prove branch conditions:
tryPushBranch() was sometimes leaving a checkpoint on the factsTable,
sometimes not, and the caller was supposed to check the return value
to know what to do.

Since we're going to make the prove descend logic a little bit more
complex by adding also induction variables, simplify the tryPushBranch
logic, by removing any factsTable checkpoint handling from it.

Passes toolstash -cmp.

Change-Id: Idfb1703df8a455f612f93158328b36c461560781
Reviewed-on: https://go-review.googlesource.com/104035
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>