compiler: implement comparing interfaces to nil

Comparing an interface to nil is easy, as the dynamic type is also nil.
Comparing the dynamic values (when the dynamic types match) is much
harder and depends on reflection capabilities, so is not yet implemented.

compiler.go

@@ -2219,6 +2219,17 @@ func (c *Compiler) parseBinOp(frame *Frame, binop *ssa.BinOp) (llvm.Value, error
 		} else {
 			return llvm.Value{}, errors.New("todo: unknown basic type in binop: " + typ.String())
 		}
+	case *types.Interface:
+		switch binop.Op {
+		case token.EQL, token.NEQ: // ==, !=
+			result := c.builder.CreateCall(c.mod.NamedFunction("runtime.interfaceEqual"), []llvm.Value{x, y}, "")
+			if binop.Op == token.NEQ {
+				result = c.builder.CreateNot(result, "")
+			}
+			return result, nil
+		default:
+			return llvm.Value{}, errors.New("binop on interface: " + binop.Op.String())
+		}
 	case *types.Pointer:
 		switch binop.Op {
 		case token.EQL: // ==