[llvm] [LangRef] Clarify that the pointer after an objet must be valid. (PR #127892)

[Nikita Popov via llvm-commits]

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@@ -11722,8 +11722,9 @@ As a corollary, the only pointer in bounds of the null pointer in the default
 address space is the null pointer itself.
 
 These rules are based on the assumption that no allocated object may cross
-the unsigned address space boundary, and no allocated object may be larger
-than half the pointer index type space.
+the unsigned address space boundary, the pointer after the object must be valid,
+and no allocated object may be larger than half the pointer index type space
+- 1.
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nikic wrote:

I'd phrase this as "than the largest signed integer that fits into the index type". It's not super clear what exactly "half" here means, but the intention is that the size is `<= SignedMax`.

https://github.com/llvm/llvm-project/pull/127892