compiler: fix nil constant interface

A nil interface has no dynamic type (or: nil dynamic type). Don't try to
use the static type as the dynamic type, because these are different.

passes.go

@@ -103,7 +103,7 @@ func (p *Program) AnalyseCallgraph() {
 // Find all types that are put in an interface.
 func (p *Program) AnalyseInterfaceConversions() {
 	// Clear, if AnalyseTypes has been called before.
-	p.typesWithoutMethods = map[string]int{"interface{}": 0, "error": 1}
+	p.typesWithoutMethods = map[string]int{"nil": 0}
 	p.typesWithMethods = map[string]*InterfaceType{}
 
 	for _, f := range p.Functions {