FIR Completion: Fix completion in function calls (foo.bar<caret>())

In such expressions, `KtCallExpression` wraps `KtSimpleNameExpression`,
so `getQualifiedExpressionForSelector()` returns `null`

Also, enable tests that are fixed by this

idea/idea-completion/testData/basic/common/AfterIntSeparatedWithComments.kt

@@ -1,3 +1,5 @@
+// FIR_COMPARISON
+
 infix fun Int.func(s: String): Int{}
 
 fun test() {