Given a pointer operand `ptr` and a signed integer operand `idx`

`ptr + idx` and `idx + ptr` -> ptr + @bitCast(usize, @intCast(isize, idx))
`ptr - idx` -> ptr - @bitCast(usize, @intCast(isize, idx))

Thanks @LemonBoy for pointing out that we can take advantage of wraparound
to dramatically simplify the code.