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getelementptr inbounds clarifications (#65478)

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- For a long time I assumed that `inbounds` means "in-bounds of a *live*
allocation". @nikic told me that is not correct. I think this definitely
needs clarification in the docs.
- The point about successively adding the offsets to the current address
confused be because it talked about the successive addition of "an"
offset -- which one? My interpretation was, the total accumulated offset
computed in the previous step. But @nikic told me that's not correct,
adding each offset individually has to stay in-bounds for each step. I
hope by saying "each offset" this becomes more clear; I then also change
the previous bullet to use the same terminology.
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llvm/docs/LangRef.rst
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@ -10955,15 +10955,17 @@ If the ``inbounds`` keyword is present, the result value of a
:ref:`poison value <poisonvalues>` if one of the following rules is violated:
* The base pointer has an *in bounds* address of an allocated object, which
means that it points into an allocated object, or to its end.
means that it points into an allocated object, or to its end. Note that the
object does not have to be live anymore; being in-bounds of a deallocated
object is sufficient.
* If the type of an index is larger than the pointer index type, the
truncation to the pointer index type preserves the signed value.
* The multiplication of an index by the type size does not wrap the pointer
index type in a signed sense (``nsw``).
* The successive addition of offsets (without adding the base address) does
* The successive addition of each offset (without adding the base address) does
not wrap the pointer index type in a signed sense (``nsw``).
* The successive addition of the current address, interpreted as an unsigned
number, and an offset, interpreted as a signed number, does not wrap the
number, and each offset, interpreted as a signed number, does not wrap the
unsigned address space and remains *in bounds* of the allocated object.
As a corollary, if the added offset is non-negative, the addition does not
wrap in an unsigned sense (``nuw``).