[llvm] [NFC ]Add a helper function isTailCall for getting libcall in SelectionDAG (PR #155256)

[Matt Arsenault via llvm-commits]

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@@ -9012,6 +9012,14 @@ static void checkAddrSpaceIsValidForLibcall(const TargetLowering *TLI,
   }
 }
 
+static bool isTailCall(const CallInst *CI, const SelectionDAG *SelDAG,
+                       bool IsLowerToLibCall) {
+  bool ReturnsFirstArg = CI && funcReturnsFirstArgOfCall(*CI);
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arsenm wrote:

On second thought, why do you need the IR analysis for funcReturnsFirstArgOfCall? You know that based on the properties of the known libcall without looking at the IR 

https://github.com/llvm/llvm-project/pull/155256