JVM_IR: fix a bug when isInlineParameter is applied to default stubs

If an inline parameter has a default value, its type is nullable.
There's already code to handle this in `IrInlineCodegen`, but it
really should be in `isInlineParameter` instead, otherwise e.g.
SyntheticAccessorLowering fails.

compiler/tests/org/jetbrains/kotlin/codegen/BlackBoxInlineCodegenTestGenerated.java

@@ -4550,6 +4550,11 @@ public class BlackBoxInlineCodegenTestGenerated extends AbstractBlackBoxInlineCo
                 runTest("compiler/testData/codegen/boxInline/syntheticAccessors/withinInlineLambda/privateInCrossInline.kt");
             }
 
+            @TestMetadata("privateInDefaultStubArgument.kt")
+            public void testPrivateInDefaultStubArgument() throws Exception {
+                runTest("compiler/testData/codegen/boxInline/syntheticAccessors/withinInlineLambda/privateInDefaultStubArgument.kt");
+            }
+
             @TestMetadata("protectedInCrossinline.kt")
             public void testProtectedInCrossinline() throws Exception {
                 runTest("compiler/testData/codegen/boxInline/syntheticAccessors/withinInlineLambda/protectedInCrossinline.kt");