[LangRef] Always allow getelementptr inbounds with zero offset

Currently, our GEP specification has a special case that makes
gep inbounds (null, 0) legal. This patch proposes to expand this
special case to all gep inbounds (ptr, 0), where ptr is no longer
required to point to an allocated object.

This was previously discussed in some detail at
https://discourse.llvm.org/t/question-about-getelementptr-inbounds-with-offset-0/62533.

The motivation for this change is twofold:

 * Rust relies on getelementptr inbounds with zero offset to be
   legal for arbitrary pointers to support zero-sized types. The
   current rules are unclear on whether this is legal or not
   (saying that there is a zero-size "allocated object" at every
   address may be consistent with our current rules, but more
   clarity is desired here).
 * The current semantics require us to drop the inbounds flag
   when materializing zero-index GEPs, which is done by some
   InstCombine transforms. Preserving the inbounds flag can
   substantially improve optimization quality in some cases, as
   illustrated in D154055.

As far as I know, the only analysis/transforms affected by this
semantics change are:

 * A special-case for comparisons with null in CaptureTracking,
   which is fixed by D154054. As far as I can tell, that special
   case is not particularly valuable and should be recovered by
   other transforms.
 * Folding gep inbounds undef, idx to poison. We now need to fold
   to undef instead (D154215).

Differential Revision: https://reviews.llvm.org/D154051