The wasm archtecture was missing a rule to handle OffPtr with a
negative offset. This commit makes it so OffPtr always gets lowered
to I64AddConst.

Fixes #25741

Change-Id: I1d48e2954e3ff31deb8cba9a9bf0cab7c4bab71a
Reviewed-on: https://go-review.googlesource.com/116595Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>