go/ssa: simplify TypesWithMethodSets

Details:
- rename (*Program).TypesWithMethodSets() to RuntimeTypes()
- delete (*Package).TypesWithMethodSets() method and simplify
- move code to methods.go
- update test to use

1-2% improvement in space and time (though I barely trust this data
because the GC at tip is in such terrible state).

Change-Id: I38eab78b11e0ad0ff16e0530e775b6ff6a2ab246
Reviewed-on: https://go-review.googlesource.com/3148
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/go/pointer/gen.go b/go/pointer/gen.go
index 48ca368..6c256ac 100644
--- a/go/pointer/gen.go
+++ b/go/pointer/gen.go
@@ -1262,7 +1262,7 @@
 
 	// Create nodes and constraints for all methods of all types
 	// that are dynamically accessible via reflection or interfaces.
-	for _, T := range a.prog.TypesWithMethodSets() {
+	for _, T := range a.prog.RuntimeTypes() {
 		a.genMethodsOf(T)
 	}