[llvm] [ARM/X86] Standardize the isEligibleForTailCallOptimization prototypes (PR #90688)

[Arthur Eubanks via llvm-commits]

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@@ -2985,14 +2984,19 @@ bool MatchingStackOffset(SDValue Arg, unsigned Offset, ISD::ArgFlagsTy Flags,
 
 /// IsEligibleForTailCallOptimization - Check whether the call is eligible
 /// for tail call optimization. Targets which want to do tail call
-/// optimization should implement this function.
+/// optimization should implement this function. Note that this function also
+/// processes musttail calls, in which case any case where this function returns
+/// true for valid musttail call results in a fatal backend error.
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aeubanks wrote:

"returns false"?

https://github.com/llvm/llvm-project/pull/90688