cmd/compile: fix constant pointer comparison failure

Previously, constant pointer-typed expressions could use either Mpint
or NilVal as their Val depending on their construction, but const.go
expects each type to have a single corresponding Val kind.

This CL changes pointer-typed expressions to exclusively use Mpint.

Fixes #21221.

Change-Id: I6ba36c9b11eb19a68306f0b296acb11a8c254c41
Reviewed-on: https://go-review.googlesource.com/105315
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -331,26 +331,11 @@ func convlit1(n *Node, t *types.Type, explicit bool, reuse canReuseNode) *Node {
 		case TARRAY:
 			goto bad
 
-		case TPTR32,
-			TPTR64,
-			TINTER,
-			TMAP,
-			TCHAN,
-			TFUNC,
-			TSLICE,
-			TUNSAFEPTR:
-			break
+		case TPTR32, TPTR64, TUNSAFEPTR:
+			n.SetVal(Val{new(Mpint)})
 
-		// A nil literal may be converted to uintptr
-		// if it is an unsafe.Pointer
-		case TUINTPTR:
-			if n.Type.Etype == TUNSAFEPTR {
-				i := new(Mpint)
-				i.SetInt64(0)
-				n.SetVal(Val{i})
-			} else {
-				goto bad
-			}
+		case TCHAN, TFUNC, TINTER, TMAP, TSLICE:
+			break
 		}
 
 	case CTSTR, CTBOOL: