[llvm] [IR] Add elementwise modifier to atomic loads (PR #204556)

[Yonah Goldberg via llvm-commits]

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@@ -12037,11 +12037,20 @@ If the ``load`` is marked as ``atomic``, it takes an extra :ref:`ordering
 Atomic loads produce :ref:`defined <memmodel>` results when they may see
 multiple atomic stores. The type of the pointee must be an integer, pointer,
 floating-point, or vector type whose bit width is a power of two greater than
-or equal to eight. ``align`` must be
-explicitly specified on atomic loads. Note: if the alignment is not greater or
-equal to the size of the `<value>` type, the atomic operation is likely to
-require a lock and have poor performance. ``!nontemporal`` does not have any
-defined semantics for atomic loads.
+or equal to eight.
+
+If the ``elementwise`` modifier is present, the loaded type must be a fixed
+vector type, and each element type must be a valid scalar atomic load type. The
+load has per-element vector atomic semantics: it behaves as if it were expanded
+into one scalar atomic load per element, and the element loads are not ordered
+with respect to each other. Without ``elementwise``, vector atomic loads keep
+whole-value atomic semantics.
+
+``align`` must be explicitly specified on atomic loads. Note: if the alignment
----------------
YonahGoldberg wrote:

Good to specify align is required. I guess this is true but we didn't state it explicitly before.

https://github.com/llvm/llvm-project/pull/204556