compiler: fix interface lowering miscompilation with reflect

When using reflect, arbitrary types can be synthesized. This invalidates
a few assumptions in the interface-lowering pass, that think they can
see all types that are in use in a program and optimize accordingly.

The file size impact depends on the specific program. Sometimes it's
nonexistent, sometimes it's rather hefty (up to 30% bigger). Especially
the samd21 targets seem to be affected, with a 2000-6000 bytes increase
in code size. A moderately large case (the stdlib test) increases by
4%/6%/15% depending on the target.

I hope that this increase could be mitigated, but I don't see an obvious
way to do that.

testdata/reflect.go

@@ -254,6 +254,16 @@ func main() {
 	if rv.Len() != 2 || rv.Index(0).Int() != 3 {
 		panic("slice was changed while setting part of it")
 	}
+
+	// Test types that are created in reflect and never created elsewhere in a
+	// value-to-interface conversion.
+	v := reflect.ValueOf(new(unreferencedType))
+	switch v.Elem().Interface().(type) {
+	case unreferencedType:
+		println("type assertion succeeded for unreferenced type")
+	default:
+		println("type assertion failed (but should succeed)")
+	}
 }
 
 func emptyFunc() {
@@ -340,3 +350,5 @@ func assertSize(ok bool, typ string) {
 		panic("size mismatch for type " + typ)
 	}
 }
+
+type unreferencedType int

testdata/reflect.txt

@@ -335,3 +335,4 @@ float32 4 32
 float64 8 64
 complex64 8 64
 complex128 16 128
+type assertion succeeded for unreferenced type