CTFE/Miri: detect out-of-bounds pointers in offset_from
Also I became uneasy with aggressively doing `try_to_int` here -- this will always succeed on Miri, leading to the wrong codepath being taken. We should rather try to convert them both to pointers, and use the integer path as a fallback, so that's what I implemented now.
Hiding whitespaces helps with the diff.
Fixes https://github.com/rust-lang/miri/issues/1950
r? ``@oli-obk``
9e70b1a033