[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/testData/codegen/box/boxingOptimization/kt48394.kt

@@ -0,0 +1,14 @@
+inline fun <R> f(size: Int, block: () -> R): R {
+    var result: R
+    while (true) {
+        result = block()
+        if (size == 0) break
+    }
+    return result
+}
+
+fun computeResult(size: Int) = f(size) {
+    42
+}
+
+fun box() = if (computeResult(0) == 42) "OK" else "FAIL"