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

[Henry Jiang via llvm-commits]

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@@ -27071,6 +27071,55 @@ None.
 This intrinsic actually does nothing, but optimizers must assume that it
 has externally observable side effects.
 
+(llvm_pseudoprobe)=
+
+#### '`llvm.pseudoprobe`' Intrinsic
+
+##### Syntax:
+
+```
+declare void @llvm.pseudoprobe(i64 <guid>, i64 <index>, i32 <attributes>, i64 <factor>) nounwind willreturn memory(inaccessiblemem: readwrite)
+```
+
+##### Overview:
+
+The `llvm.pseudoprobe` intrinsic is a placeholder for the block that
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mustartt wrote:

Maybe something along the lines of
> The llvm.pseudoprobe intrinsic identify the _basic block_ of the function's pre-optimized CFG.

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