[llvm] [AA] Teach getModRefInfo(FenceInst) to consult the AA chain (PR #192043)

[Matt Arsenault via llvm-commits]

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@@ -7418,9 +7418,13 @@ does not carry useful data and need not be preserved.
 noalias memory-access sets. This means that some collection of memory access
 instructions (loads, stores, memory-accessing calls, etc.) that carry
 ``noalias`` metadata can specifically be specified not to alias with some other
-collection of memory access instructions that carry ``alias.scope`` metadata. If
-accesses from different collections alias, the behavior is undefined. Each type
-of metadata specifies a list of scopes where each scope has an id and a domain.
+collection of memory access instructions that carry ``alias.scope`` metadata.
+These metadata kinds may also be attached to ``fence`` instructions to indicate
+which scoped memory regions the fence does (or does not) concern; this allows
+alias analysis to prove that a fence cannot affect a particular memory location.
+If accesses from different collections alias, the behavior is undefined. Each
+type of metadata specifies a list of scopes where each scope has an id and a
+domain.
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arsenm wrote:

LangRef change looks unrelated and isn't backed by any other change? 

https://github.com/llvm/llvm-project/pull/192043