Do not treat uninitialized value as a reason to retain boxing

See testData/simpleUnitializedMerge.kt
On exit from `if` we merge value in variable with slot 1 (x):
- from `if` body we get BoxedBasicValue
- from outer block we get UNITIALIZED_VALUE

So we just suppose `x` is unitialized after `if`
and there's no need to mark BoxedValue as unsafe to remove
because it's anyway can't be used after `if`

 #KT-6842 Fixed

compiler/testData/codegen/bytecodeText/boxingOptimization/simpleUninitializedMerge.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    var result = 0
+    if (1 == 1) {
+        val x: Int? = 1
+        result += x!!
+    }
+    return "OK"
+}
+
+// 0 java/lang/Integer.valueOf