[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/tests-gen/org/jetbrains/kotlin/codegen/LightAnalysisModeTestGenerated.java

@@ -1510,6 +1510,11 @@ public class LightAnalysisModeTestGenerated extends AbstractLightAnalysisModeTes
             runTest("compiler/testData/codegen/box/boxingOptimization/kt46859.kt");
         }
 
+        @TestMetadata("kt48394.kt")
+        public void testKt48394() throws Exception {
+            runTest("compiler/testData/codegen/box/boxingOptimization/kt48394.kt");
+        }
+
         @TestMetadata("kt5493.kt")
         public void testKt5493() throws Exception {
             runTest("compiler/testData/codegen/box/boxingOptimization/kt5493.kt");