GNU Compiler Collection (GCC) Internals: Using poly_int with C++ arithmetic operators

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#### 10.4.1 Using `poly_int` with C++ arithmetic operators

The following C++ expressions are supported, where p1 and p2 are `poly_int`s and where c1 and c2 are scalars:

    -p1
    ~p1

    p1 + p2
    p1 + c2
    c1 + p2

    p1 - p2
    p1 - c2
    c1 - p2

    c1 * p2
    p1 * c2

    p1 << c2

    p1 += p2
    p1 += c2

    p1 -= p2
    p1 -= c2

    p1 *= c2
    p1 <<= c2

These arithmetic operations handle integer ranks in a similar way to C++. The main difference is that every coefficient narrower than `HOST_WIDE_INT` promotes to `HOST_WIDE_INT`, whereas in C++ everything narrower than `int` promotes to `int`. For example:

    poly_uint16     + int          -> poly_int64
    unsigned int    + poly_uint16  -> poly_int64
    poly_int64      + int          -> poly_int64
    poly_int32      + poly_uint64  -> poly_uint64
    uint64          + poly_int64   -> poly_uint64
    poly_offset_int + int32        -> poly_offset_int
    offset_int      + poly_uint16  -> poly_offset_int

In the first two examples, both coefficients are narrower than `HOST_WIDE_INT`, so the result has coefficients of type `HOST_WIDE_INT`. In the other examples, the coefficient with the highest rank “wins”.

If one of the operands is `wide_int` or `poly_wide_int`, the rules are the same as for `wide_int` arithmetic.