cmd/compile: delay fillinMethods to deal with mutually-recursive types

We need to delay fillinMethods until we get to a top-level type, so we
know all the TFORW types have been filled in, and we can do the
substitutions required by fillinMethods.

Fixes #47710

Change-Id: I298de7e7753ed31a2c2b1ff04f35177a8afc7a66
Reviewed-on: https://go-review.googlesource.com/c/go/+/345149
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -149,6 +149,9 @@ type irgen struct {
 	// statements yet.
 	exprStmtOK bool
 
+	// types which we need to finish, by doing g.fillinMethods.
+	typesToFinalize []*typeDelayInfo
+
 	// Fully-instantiated generic types whose methods should be instantiated
 	instTypeList []*types.Type
 
@@ -184,6 +187,11 @@ type delayInfo struct {
 	off   int
 }
 
+type typeDelayInfo struct {
+	typ  *types2.Named
+	ntyp *types.Type
+}
+
 func (g *irgen) generate(noders []*noder) {
 	types.LocalPkg.Name = g.self.Name()
 	types.LocalPkg.Height = g.self.Height()