[llvm] Add reference of llvm.pseudoprobe in LangRef.md (PR #223065)

[Matthias Braun via llvm-commits]

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@@ -27083,42 +27083,60 @@ declare void @llvm.pseudoprobe(i64 <guid>, i64 <index>, i32 <attributes>, i64 <f
 
 ##### Overview:
 
-The `llvm.pseudoprobe` intrinsic is a placeholder for the block that
-contains it. It is emitted for sample-based profile-guided optimization, so
-that samples collected from an optimized binary can be attributed back to a
-point in the original program. It performs no operation.
+The `llvm.pseudoprobe` intrinsic identifies a basic block of the function's
+pre-optimized CFG, so that samples collected from an optimized binary can be
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MatzeB wrote:

I like the framing as "pre-optimized" but should probably be something like  "... identifies a basic block in a function before the module was optimized" instead of CFG.   (as probes will also follow-along as functions get inlined and may originally have belonged to a different function where it is now ...).

https://github.com/llvm/llvm-project/pull/223065