[llvm] [AMDGPU] Remove duplicated/confusing helpers. NFCI (PR #142598)

[Shilei Tian via llvm-commits]

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@@ -1343,6 +1340,10 @@ inline bool isKernel(CallingConv::ID CC) {
   }
 }
 
+/// Return true if we might ever do TCO for calls with this calling convention.
+bool mayTailCallThisCC(CallingConv::ID CC);
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shiltian wrote:

They look small enough to be a header only implementation. They can be a `constexpr` as well.

https://github.com/llvm/llvm-project/pull/142598