JVM_IR: do not explicitly initialize Refs to default values

compiler/testData/codegen/bytecodeText/capturedVarsOptimization/sharedSlotsWithCapturedVars.kt

@@ -1,6 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
-// TODO KT-36648 Captured variables not optimized in JVM_IR
-
 fun box(): String {
     run {
         run {
@@ -24,8 +21,7 @@ fun box(): String {
 // 4 ILOAD 6
 // 4 ISTORE 6
 
-// Temporary variable slots for 'x2++' + store to fake index:
-// 0 ILOAD 1
+// Temporary variable for 'x2++' + store to fake variable marking the outer `run`:
 // 2 ISTORE 1
 
 // 0 NEW