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/box/contracts/exactlyOnceNotInline.kt

@@ -1,6 +1,6 @@
 // !LANGUAGE: +AllowContractsForCustomFunctions +UseCallsInPlaceEffect +ReadDeserializedContracts
 // !USE_EXPERIMENTAL: kotlin.contracts.ExperimentalContracts
-// IGNORE_BACKEND: JVM_IR, NATIVE, JS_IR
+// IGNORE_BACKEND: NATIVE
 
 import kotlin.contracts.*
 
@@ -18,4 +18,4 @@ fun box(): String {
         x = 1L
     }
     return if (x != 1L) "FAIL" else "OK"
-}
+}