Avoid V for unbalanced stack if single possible path

[ckaestne]

There are several cases where we may avoid using a V for variables storing unbalanced stack entries.
If we split on a variational if statement, it should be sufficient to keep the old plain value, since it is read again directly (possibly on a more narrow context) and cannot have alternative values in the meantime

I think the following condition needs to apply for this to happen: There is only a single path from the beginning of the method to this statement, i.e. no loops in the CFG to come back to this statement.

@chupanw