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/testData/codegen/boxInline/syntheticAccessors/withinInlineLambda/privateInDefaultStubArgument.kt

@@ -0,0 +1,18 @@
+// FILE: 1.kt
+package test
+
+// Argument `f` of `call$default` is technically nullable and inline.
+inline fun call(other: Int = 1, crossinline f: () -> String = { "fail" }) = { f() }()
+
+// FILE: 2.kt
+import test.*
+
+class A {
+    private fun method() = "O"
+
+    private val prop = "K"
+
+    fun test(): String = call { method() + prop }
+}
+
+fun box(): String = A().test()