cmd/internal/obj/wasm: avoid invalid offsets for Load/Store

Offsets for Load and Store instructions have type i32. Bad index
expression offsets can cause an offset to be larger than MaxUint32,
which is not allowed. One example for this is the test test/index0.go.

Generate valid code by adding a guard to the responsible rewrite rule.
Also emit a proper error when using such a bad index in assembly code.

Change-Id: Ie90adcbf3ae3861c26680eb81790f28692913ccf
Reviewed-on: https://go-review.googlesource.com/111955


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>