[llvm] [IR] Initial introduction of memset_pattern (PR #97583)

[Alex Bradbury via llvm-commits]

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@@ -1003,6 +1003,14 @@ def int_memset_inline
        NoCapture<ArgIndex<0>>, WriteOnly<ArgIndex<0>>,
        ImmArg<ArgIndex<3>>]>;
 
+// Memset variant that writes a given pattern.
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asb wrote:

Added with inline comments (there's not a totally consistent pattern for describing args - many intrinsics have no description at all, but I see some other examples in the file using inline comments for the args)

https://github.com/llvm/llvm-project/pull/97583