cmd/compile: canonicalize empty interface types

Mapping all empty interfaces onto the same Type
allows better reuse of the ptrTo and sliceOf
Type caches for *interface{} and []interface{}.

This has little compiler performance impact now,
but it will be helpful in the future,
when we will eagerly populate some of those caches.

Passes toolstash-check.

Change-Id: I17daee599a129b0b2f5f3025c1be43d569d6782c
Reviewed-on: https://go-review.googlesource.com/38344
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -526,11 +526,15 @@ func (p *importer) typ() *Type {
 		functypefield0(t, nil, params, result)
 
 	case interfaceTag:
-		t = p.newtyp(TINTER)
 		if p.int() != 0 {
 			formatErrorf("unexpected embedded interface")
 		}
-		t.SetFields(p.methodList())
+		if ml := p.methodList(); len(ml) == 0 {
+			t = Types[TINTER]
+		} else {
+			t = p.newtyp(TINTER)
+			t.SetFields(ml)
+		}
 		checkwidth(t)
 
 	case mapTag:

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

@@ -860,6 +860,9 @@ func interfacefield(n *Node) *Field {
 }
 
 func tointerface(l []*Node) *Type {
+	if len(l) == 0 {
+		return Types[TINTER]
+	}
 	t := typ(TINTER)
 	tointerface0(t, l)
 	return t