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

[Matt Arsenault via llvm-commits]

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@@ -2185,8 +2185,16 @@ bool isChainCC(CallingConv::ID CC) {
   }
 }
 
-bool isKernelCC(const Function *Func) {
-  return AMDGPU::isModuleEntryFunctionCC(Func->getCallingConv());
+bool canGuaranteeTCO(CallingConv::ID CC) { return CC == CallingConv::Fast; }
+
+bool mayTailCallThisCC(CallingConv::ID CC) {
+  switch (CC) {
+  case CallingConv::C:
+  case CallingConv::AMDGPU_Gfx:
+    return true;
+  default:
+    return canGuaranteeTCO(CC);
+  }
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arsenm wrote:

Might as well keep these in the header 

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