[FIR] Unify some special names to make code more uniform

compiler/testData/ir/irText/expressions/arrayAugmentedAssignment2.fir.kt.txt

@@ -10,8 +10,8 @@ interface IB {
 
 fun IB.test(a: IA) {
   { // BLOCK
-    val <<array>>: IA = a
-    val <<index_0>>: String = ""
-    (<this>, <<array>>).set(index = <<index_0>>, value = <<array>>.get(index = <<index_0>>).plus(other = 42))
+    val <array>: IA = a
+    val <index_0>: String = ""
+    (<this>, <array>).set(index = <index_0>, value = <array>.get(index = <index_0>).plus(other = 42))
   }
 }