[llvm] [AArch64][PAC] Fix creating check instructions for BBs without an epilog (PR #92508)

[Anatoly Trosinenko via llvm-commits]

================
@@ -246,65 +246,56 @@ MachineBasicBlock &llvm::AArch64PAuth::checkAuthenticatedRegister(
   default:
     break;
   case AuthCheckMethod::None:
-    return MBB;
+    return;
   case AuthCheckMethod::DummyLoad:
     BuildMI(MBB, MBBI, DL, TII->get(AArch64::LDRWui), getWRegFromXReg(TmpReg))
         .addReg(AuthenticatedReg)
         .addImm(0)
         .addMemOperand(createCheckMemOperand(MF, Subtarget));
-    return MBB;
+    return;
   }
 
   // Control flow has to be changed, so arrange new MBBs.
 
-  // At now, at least an AUT* instruction is expected before MBBI
-  assert(MBBI != MBB.begin() &&
-         "Cannot insert the check at the very beginning of MBB");
-  // The block to insert check into.
-  MachineBasicBlock *CheckBlock = &MBB;
-  // The remaining part of the original MBB that is executed on success.
-  MachineBasicBlock *SuccessBlock = MBB.splitAt(*std::prev(MBBI));
-
   // The block that explicitly generates a break-point exception on failure.
   MachineBasicBlock *BreakBlock =
       MF.CreateMachineBasicBlock(MBB.getBasicBlock());
   MF.push_back(BreakBlock);
-  MBB.splitSuccessor(SuccessBlock, BreakBlock);
+  MBB.addSuccessor(BreakBlock);
 
-  assert(CheckBlock->getFallThrough() == SuccessBlock);
   BuildMI(BreakBlock, DL, TII->get(AArch64::BRK)).addImm(BrkImm);
 
----------------
atrosinenko wrote:

Using non-terminator instruction as MBBI would introduce a jump to `BreakBlock` before non-terminator instruction. On the other hand, using not the **first** terminator as MBBI seems incorrect as well. Anyway, the patch as-is looks good to me.

https://github.com/llvm/llvm-project/pull/92508