Reuse findIndVar to discover induction variables, and then
register the facts we know about them into the facts table
when entering the loop block.

Moreover, handle "x+delta > w" while updating the facts table,
to be able to prove accesses to slices with constant offsets
such as slice[i-10].

Change-Id: I2a63d050ed58258136d54712ac7015b25c893d71
Reviewed-on: https://go-review.googlesource.com/104038
Run-TryBot: Giovanni Bajo <rasky@develer.com>
Reviewed-by: David Chase <drchase@google.com>