cmd/compile: fix the index variable is shadowed in dictPass

The CL 349613 causes this problem.
In fact, we want to use the outer i to find m.List[i],
but the newly created index variable i in the nearest
for range shadow the outer i.

Fixes #48838.

Change-Id: I10f0bd985340f9443eefaadda6fc56e4e7e9a10c
Reviewed-on: https://go-review.googlesource.com/c/go/+/354549
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>

src/cmd/compile/internal/noder/stencil.go

@@ -1298,9 +1298,9 @@ func (g *irgen) dictPass(info *instInfo) {
 							// Type switch from nonempty interface. We need a *runtime.itab
 							// for the dynamic type.
 							ix := -1
-							for i, ic := range info.dictInfo.itabConvs {
+							for j, ic := range info.dictInfo.itabConvs {
 								if ic == m.List[i] {
-									ix = info.dictInfo.startItabConv + i
+									ix = info.dictInfo.startItabConv + j
 									break
 								}
 							}

test/typeparam/issue48838.go

@@ -0,0 +1,31 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 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
+
+func main() {
+	check[string]()
+}
+
+func check[T any]() {
+	var result setter[T]
+	switch result.(type) {
+	case fooA[T]:
+	case fooB[T]:
+	}
+}
+
+type setter[T any] interface {
+	Set(T)
+}
+
+type fooA[T any] struct{}
+
+func (fooA[T]) Set(T) {}
+
+type fooB[T any] struct{}
+
+func (fooB[T]) Set(T) {}