Revert "[FIR] Remove the redundant isFunctionType check"

This reverts commit 1f97c268e1.

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

@@ -1,12 +0,0 @@
-FILE: castForFunctionsWithDifferentArities.kt
-    public final fun test(b: R|kotlin/Boolean|, block1: R|kotlin/Any.() -> kotlin/Unit|, block2: R|(kotlin/Any.(kotlin/Any?) -> kotlin/Unit)?|): R|kotlin/Unit| {
-        when () {
-            R|<local>/b| ->  {
-                R|kotlin/requireNotNull|<R|kotlin/Any.() -> kotlin/Unit|>(R|<local>/block1|)
-            }
-            else ->  {
-                R|kotlin/requireNotNull|<R|kotlin/Any.(kotlin/Any?) -> kotlin/Unit|>(R|<local>/block2|)
-            }
-        }
-
-    }

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)
-    }
-}