Don't adjust hp up when the case scrutinee won't allocate

Don't adjust hp up when the case scrutinee won't allocate

If the case scrutinee can't allocate, we don't need to do a heap
check in the case alternatives. (A previous patch got that right.)
In that case, we had better not adjust the heap pointer to recover
unused stack space before evaluating the scrutinee -- because we
aren't going to reallocate for the case alternative.