IR: create shared variables for val-variables when needed

This is possible when a lambda's contract guarantees initialization of a
variable.

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

@@ -0,0 +1,18 @@
+// WITH_RUNTIME
+// IGNORE_BACKEND: JVM_IR
+
+// In JVM IR, SharedVariablesLowering transforms `x` into a shared variable to be able to update it from a lambda,
+// which is a separate function (...$lambda-0).
+// If we keep the existing representation of lambda bodies as separate functions in JVM IR, the only viable option to fix this test
+// seems to support this case in the bytecode optimization pass CapturedVarsOptimizationMethodTransformer.
+
+fun box(): String {
+    val x: String
+    run {
+        x = "OK"
+        val y = x
+    }
+    return x
+}
+
+// 0 ObjectRef

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

@@ -0,0 +1,10 @@
+fun box(): String {
+    val x: String
+    x = "OK"
+    {
+        val y = x
+    }()
+    return x
+}
+
+// 0 ObjectRef