cmd/compile: add missing StringLen rule in prove

cmd/compile: add missing StringLen rule in prove

(StringLen (StringMake _ x)) == x, just like the rules we
currently have for slices.

This helps propagate string length knowledge to places which need it.

Change-Id: Ifdcf6d1f2d430c1c4bbac32e0ea74c188eae998e
Reviewed-on: https://go-review.googlesource.com/c/go/+/682777
Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>