cmd/compile: allow converting defined string types to []rune

Fixes #23298

Change-Id: I107c6f3a80db83f063c0daf262c6e7f7492e4d4c
Reviewed-on: https://go-review.googlesource.com/87695
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/walk.go

@@ -1665,7 +1665,7 @@ opswitch:
 			a = nod(OADDR, temp(t), nil)
 		}
 
-		n = mkcall("stringtoslicerune", n.Type, init, a, n.Left)
+		n = mkcall("stringtoslicerune", n.Type, init, a, conv(n.Left, types.Types[TSTRING]))
 
 		// ifaceeq(i1 any-1, i2 any-2) (ret bool);
 	case OCMPIFACE:

test/fixedbugs/issue23298.go

+// 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 p
+
+type T string
+
+var (
+	t = T("T")
+	r = []rune(t)
+)