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
