[FIR] Add lightTree support to extended checkers + minor refactorings

compiler/fir/analysis-tests/testData/extendedCheckers/RedundantVisibilityModifierChecker.kt

@@ -1,10 +1,9 @@
 fun f() {
-
+    <!REDUNDANT_VISIBILITY_MODIFIER!>public<!> <!CAN_BE_VAL!>var<!> baz = 0
     class LocalClass {
         <!REDUNDANT_VISIBILITY_MODIFIER!>internal<!> var foo = 0
     }
     LocalClass().foo = 1
-    <!REDUNDANT_VISIBILITY_MODIFIER!>public<!> <!CAN_BE_VAL!>var<!> baz = 0
 }
 
 internal inline fun internal() {

compiler/fir/analysis-tests/testData/extendedCheckers/RedundantVisibilityModifierChecker.txt

@@ -1,5 +1,6 @@
 FILE: RedundantVisibilityModifierChecker.kt
     public final fun f(): R|kotlin/Unit| {
+        lvar baz: R|kotlin/Int| = Int(0)
         local final class LocalClass : R|kotlin/Any| {
             public constructor(): R|LocalClass| {
                 super<R|kotlin/Any|>()
@@ -12,7 +13,6 @@ FILE: RedundantVisibilityModifierChecker.kt
         }
 
         R|/LocalClass.LocalClass|().R|/LocalClass.foo| = Int(1)
-        lvar baz: R|kotlin/Int| = Int(0)
     }
     internal final inline fun internal(): R|kotlin/Unit| {
         R|/f|()