llvm dev - Nov 2020 - Wrapping rules for GEP inbounds

Hi,

I've put up https://reviews.llvm.org/D90708 to clarify the wrapping
limitations for GEP inbounds, because I find the current wording ambiguous.
I think this codifies the way it is currency interpreted in practice, but
I'd like to have some wider feedback on that.

Essentially this comes down to two questions:

 * Can an allocated object cross the unsigned address space boundary? (In
address space zero the answer is clearly "no", due to the null
pointer.)
 * Can an allocated object cross the signed address space boundary? (On
x86-64 the answer is clearly "no", due to canonical address space
layout.)

My current assumption is that the answer to the first question is "no"
and
the answer to the second question is "yes". (For optimization, it
would be
ideal if both questions were answered by "yes".)

Regards,
Nikita
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