cmd/compile: derive len/cap relations in factsTable.update

Currently, the prove pass derives implicit relations between len and
cap in the code that adds branch conditions. This is fine right now
because that's the only place we can encounter len and cap, but we're
about to add a second way to add assertions to the facts table that
can also produce facts involving len and cap.

Prepare for this by moving the fact derivation from updateRestrictions
(where it only applies on branches) to factsTable.update, which can
derive these facts no matter where the root facts come from.

Passes toolstash -cmp.

Change-Id: If09692d9eb98ffaa93f4cfa58ed2d8ba0887c111
Reviewed-on: https://go-review.googlesource.com/102602


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>