Revert "[FIR] Remove the redundant isFunctionType check"

This reverts commit 1f97c268e1.

compiler/fir/analysis-tests/testData/resolveWithStdlib/castForFunctionsWithDifferentArities.kt

@@ -1,9 +0,0 @@
-// ISSUE: KT-63865
-
-fun test(b: Boolean, block1: Any.() -> Unit, block2: (Any.(Any?) -> Unit)?) {
-    if (b) {
-        requireNotNull(block1)
-    } else {
-        requireNotNull(block2)
-    }
-}