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.

compiler.go

@@ -2279,16 +2279,10 @@ func (c *Compiler) parseConst(expr *ssa.Const) (llvm.Value, error) {
 		if expr.Value != nil {
 			return llvm.Value{}, errors.New("non-nil interface constant")
 		}
-		itfTypeNum, ok := c.ir.TypeNum(expr.Type())
-		if itfTypeNum >= 1<<16 {
-			return llvm.Value{}, errors.New("interface typecodes do not fit in a 16-bit integer")
-		}
-		if !ok {
-			panic("interface number is unknown")
-		}
+		// Create a generic nil interface with no dynamic type (typecode=0).
 		fields := []llvm.Value{
-			llvm.ConstInt(llvm.Int16Type(), uint64(itfTypeNum), false),
-			llvm.Undef(c.i8ptrType),
+			llvm.ConstInt(llvm.Int16Type(), 0, false),
+			llvm.ConstPointerNull(c.i8ptrType),
 		}
 		itf := llvm.ConstNamedStruct(c.mod.GetTypeByName("runtime._interface"), fields)
 		return itf, nil

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 {