Allow invertibility for unchecked pointers

[kkjeer]

We should calculate original values for unchecked pointers in invertible expressions. For example:

#pragma CHECKED_SCOPE off
void f(char * p : bounds(p, p + 1)) {
  p = p + 1;
}

Since the body of f is in a unchecked scope, p has an unchecked pointer type char *. The original value of p in the assignment p = p + 1 should be p - 1.