cmd/compile: add cases for StringLen to prove

cmd/compile: add cases for StringLen to prove

Tricky index-offset logic had been added for slices,
but not for strings.  This fixes that, and also adds
tests for same behavior in string/slice cases, and adds
a new test for code in prove that had been added but not
explicitly tested.

Fixes #76270.

Change-Id: Ibd92b89e944d86b7f30b4486a9008e6f1ac6af7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/723980
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>