cmd/compile: do not add invalid key to constSet

After CL 272654, the compiler now use go/constant.Value to represent
constant nodes. That makes ir.ConstantValue requires node type to
correctly return value for untyped int node. But untyped int node can
have nil type after typechecked, e.g: using int value as key for
map[string]int, that makes the compiler crashes.

To fix it, just don't add the invalid key to constSet, since when
it's not important to report duplicated keys when they aren't valid.

For #43311
Fixes #44432

Change-Id: I44d8f2b95f5cb339e77e8a705a94bcb16e62beb9
Reviewed-on: https://go-review.googlesource.com/c/go/+/294034
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: cuonglm

src/cmd/compile/internal/typecheck/const.go

@@ -794,7 +794,7 @@ func (s *constSet) add(pos src.XPos, n ir.Node, what, where string) {
 		}
 	}
 
-	if !ir.IsConstNode(n) {
+	if !ir.IsConstNode(n) || n.Type() == nil {
 		return
 	}
 	if n.Type().IsUntyped() {

test/fixedbugs/issue44432.go

@@ -0,0 +1,13 @@
+// errorcheck -G=0 -d=panic
+
+// 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 p
+
+var m = map[string]int{
+	"a": 1,
+	1:   1, // ERROR "cannot use 1.*as type string in map key"
+	2:   2, // ERROR "cannot use 2.*as type string in map key"
+}