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/boxInline/contracts/exactlyOnceNoinline.kt

@@ -1,9 +1,10 @@
 // !LANGUAGE: +AllowContractsForCustomFunctions +UseCallsInPlaceEffect +ReadDeserializedContracts
 // !USE_EXPERIMENTAL: kotlin.contracts.ExperimentalContracts
-// IGNORE_BACKEND: JVM_IR, NATIVE, JS_IR
-// IGNORE_BACKEND_MULTI_MODULE: JVM_IR
+// IGNORE_BACKEND: NATIVE
 // NO_CHECK_LAMBDA_INLINING
+
 // FILE: 1.kt
+
 package test
 
 import kotlin.contracts.*
@@ -17,6 +18,7 @@ public inline fun myrun(noinline block: () -> Unit): Unit {
 }
 
 // FILE: 2.kt
+
 import test.*
 
 fun box(): String {
@@ -25,4 +27,4 @@ fun box(): String {
         x = 42L
     }
     return if (x != 42L) "FAIL" else "OK"
-}
+}