[llvm] [IR][Attribute] Add support for intersecting AttributeLists; NFC (PR #109719)

[Nikita Popov via llvm-commits]

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@@ -22,290 +22,323 @@ def ParamAttr : AttrProperty;
 /// Can be used as return attribute.
 def RetAttr : AttrProperty;
 
+
+
+/// Intersection rules. Used for example in sinking/hoisting two
+/// callbases to find a set of attributes that apply to both.
+/// Note, there are some attributes we can (probably) legally drop
+/// but are intentionally excluded as of now. Those include:
+///     - initializes
+///     - allockind
+///     - allocsize
+///     - minsize
+///     - optsize
+///     - optnone
+///     - optdebug
+///     - optforfuzzing
+///
+/// When intersecting the attributes must both present and equal.
+/// Use this for attributes you are not certain it is safe to drop
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nikic wrote:

```suggestion
/// Use this for attributes it is not safe to drop
```
I don't think we need to imply uncertainty here...

https://github.com/llvm/llvm-project/pull/109719