JVM_IR: do not explicitly initialize Refs to default values

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/descriptors/SharedVariablesManager.kt

@@ -107,6 +107,11 @@ class JvmSharedVariablesManager(
 
     override fun defineSharedValue(originalDeclaration: IrVariable, sharedVariableDeclaration: IrVariable): IrStatement {
         val initializer = originalDeclaration.initializer ?: return sharedVariableDeclaration
+        val default = IrConstImpl.defaultValueForType(initializer.startOffset, initializer.endOffset, originalDeclaration.type)
+        if (initializer is IrConst<*> && initializer.value == default.value) {
+            // The field is preinitialized to the default value, so an explicit set is not required.
+            return sharedVariableDeclaration
+        }
         val initializationStatement = with(initializer) {
             IrSetVariableImpl(startOffset, endOffset, irBuiltIns.unitType, originalDeclaration.symbol, this, null)
         }

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

@@ -1,5 +1,5 @@
 fun test() {
-    var x = 0
+    var x = 1
     run {
         val obj = object {
             fun foo() { ++x }

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

@@ -1,7 +1,7 @@
 fun runNoInline(f: () -> Unit) = f()
 
 fun test() {
-    var x = 0
+    var x = 1
     runNoInline { ++x }
 }
 

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

@@ -1,7 +1,7 @@
 fun runNoInline(f: () -> Unit) = f()
 
 fun test() {
-    var x = 0
+    var x = 1
     run {
         run {
             runNoInline {

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

compiler/testData/codegen/bytecodeText/localInitializationLVT/lateinit.kt

@@ -7,10 +7,4 @@ fun test(): Char {
 }
 
 // 1 LOCALVARIABLE c Ljava/lang/Object;
-
-// JVM_TEMPLATES
-// 2 ASTORE 0
-
-// JVM_IR_TEMPLATES
-// 3 ASTORE 0
-// *two* of them are after the start of c's live range
+// 2 ASTORE 0