[llvm] [IR] Require that ptrmask mask matches pointer index size (PR #69343)

[Jessica Clarke via llvm-commits]

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@@ -26965,10 +26966,20 @@ to facilitate alias analysis and underlying-object detection.
 Semantics:
 """"""""""
 
-The result of ``ptrmask(ptr, mask)`` is equivalent to
-``getelementptr ptr, (ptrtoint(ptr) & mask) - ptrtoint(ptr)``. Both the returned
-pointer(s) and the first argument are based on the same underlying object (for more
-information on the *based on* terminology see
+The result of ``ptrmask(%ptr, %mask)`` is equivalent to the following expansion,
+where ``iPtrIdx`` is the index type size of the pointer::
+
+    %intptr = ptrtoint ptr %ptr to iPtrIdx ; this may truncate
+    %masked = and iPtrIdx %intptr, %mask
+    %diff = sub iPtrIdx %masked, %intptr
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jrtc27 wrote:

Indeed, it works specifically because everything is using iPtrIdx arithmetic, and therefore (in two's complement) does not matter whether things are positive or negative, it's just modular arithmetic

https://github.com/llvm/llvm-project/pull/69343