[JVM] Do not unbox when local variable initialized with null.

This is already the case for straightline code such as

```
inline fun <R> f(size: Int, block: () -> R): R {
    var result: R
    result = block()
    return result
}
```

However, if the local variable introduction happens at a merge
point as in the following example, we allow the unboxing but
only do it halfway. The initialization of the local is still
done with a null value.

```
inline fun <R> f(size: Int, block: () -> R): R {
    var result: R
    while (true) {
        result = block()
        if (size == 0) break
    }
    return result
}
```

This change disallows unboxing for this move complicated
case as well by bailing out if a local use is with a
TaintedBoxedValue (merge of Object and Integer).

^KT-48394 Fixed.

compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/RedundantBoxingMethodTransformer.kt

@@ -129,7 +129,7 @@ class RedundantBoxingMethodTransformer(private val generationState: GenerationSt
     private fun isUnsafeToRemoveBoxingForConnectedValues(usedValues: List<BasicValue>, unboxedType: Type): Boolean =
         usedValues.any { input ->
             if (input === StrictBasicValue.UNINITIALIZED_VALUE) return@any false
-            if (input !is BoxedBasicValue) return@any true
+            if (input !is CleanBoxedValue) return@any true
 
             val descriptor = input.descriptor
             !descriptor.isSafeToRemove || descriptor.unboxedType != unboxedType