Get variable and constant from upper bounds

[kkjeer]

This PR adds a GetVariableAndConstant method to NormalizeUtil in order to compare two upper bound expressions. This enables the bounds checker to validate bounds that have been widened multiple times.

For example:

void f(_Nt_array_ptr<char> p : count(len), unsigned int len) {
  if (*(p + len)) {
    if (*(p + len + 1)) {
      // Widened bounds before increment: bounds(p, ((p + len) + 1) + 1)
      // Inferred bounds after increment: bounds(p, (((p + (len - 1)) + 1)) + 1)
     ++len;
    }
  }
}

GetVariableAndConstant separates an input expression E into a pointer-typed expression E1 and an integer constant C for as long as possible. It adds up each constant found this way and, if the result does not overflow, sets the out parameter Constant to the result of adding up all the constants and sets the out parameter Variable to the final pointer-typed subexpression of E.

[sulekhark]

LGTM! Thank you!

[mgrang]

LGTM.