cmd/compile: fix OffPtr with negative offset on wasm

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/116595
Reviewed-by: Cherry Zhang <[email protected]>
Run-TryBot: Cherry Zhang <[email protected]>

Author: neelance

src/cmd/compile/internal/ssa/gen/Wasm.rules

@@ -46,8 +46,7 @@
 (Not x) -> (I64Eqz x)
 
 // Lowering pointer arithmetic
-(OffPtr [0] ptr) -> ptr
-(OffPtr [off] ptr) && off > 0 -> (I64AddConst [off] ptr)
+(OffPtr [off] ptr) -> (I64AddConst [off] ptr)
 
 // Lowering extension
 // It is unnecessary to extend loads
@@ -388,6 +387,7 @@
 (I64Ne x (I64Const [0])) -> (I64Eqz (I64Eqz x))
 
 (I64Add x (I64Const [y])) -> (I64AddConst [y] x)
+(I64AddConst [0] x) -> x
 (I64Eqz (I64Eqz (I64Eqz x))) -> (I64Eqz x)
 
 ((I64Load|I64Load32U|I64Load32S|I64Load16U|I64Load16S|I64Load8U|I64Load8S) [off] (I64AddConst [off2] ptr) mem)

src/cmd/compile/internal/ssa/rewriteWasm.go

@@ -0,0 +1,14 @@
+// compile
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+var s []int
+
+func main() {
+	i := -1
+	s[i] = 0
+}