[LoongArch] Eliminate extra un-accounted-for successors

Specifically:
```
*** Bad machine code: MBB has unexpected successors which are not branch targets, fallthrough, EHPads, or inlineasm_br targets. ***
- function:    atomicrmw_umax_i8_acquire
- basic block: %bb.3  (0x1b90bd8)

*** Bad machine code: Non-terminator instruction after the first terminator ***
- function:    atomicrmw_umax_i8_acquire
- basic block: %bb.3  (0x1b90bd8)
- instruction: DBAR 1792
```

Differential Revision: https://reviews.llvm.org/D137884

llvm/lib/Target/LoongArch/LoongArchExpandAtomicPseudoInsts.cpp

@@ -347,20 +347,23 @@ bool LoongArchExpandAtomicPseudo::expandAtomicMinMaxOp(
   auto LoopHeadMBB = MF->CreateMachineBasicBlock(MBB.getBasicBlock());
   auto LoopIfBodyMBB = MF->CreateMachineBasicBlock(MBB.getBasicBlock());
   auto LoopTailMBB = MF->CreateMachineBasicBlock(MBB.getBasicBlock());
+  auto TailMBB = MF->CreateMachineBasicBlock(MBB.getBasicBlock());
   auto DoneMBB = MF->CreateMachineBasicBlock(MBB.getBasicBlock());
 
   // Insert new MBBs.
   MF->insert(++MBB.getIterator(), LoopHeadMBB);
   MF->insert(++LoopHeadMBB->getIterator(), LoopIfBodyMBB);
   MF->insert(++LoopIfBodyMBB->getIterator(), LoopTailMBB);
-  MF->insert(++LoopTailMBB->getIterator(), DoneMBB);
+  MF->insert(++LoopTailMBB->getIterator(), TailMBB);
+  MF->insert(++TailMBB->getIterator(), DoneMBB);
 
   // Set up successors and transfer remaining instructions to DoneMBB.
   LoopHeadMBB->addSuccessor(LoopIfBodyMBB);
   LoopHeadMBB->addSuccessor(LoopTailMBB);
   LoopIfBodyMBB->addSuccessor(LoopTailMBB);
   LoopTailMBB->addSuccessor(LoopHeadMBB);
-  LoopTailMBB->addSuccessor(DoneMBB);
+  LoopTailMBB->addSuccessor(TailMBB);
+  TailMBB->addSuccessor(DoneMBB);
   DoneMBB->splice(DoneMBB->end(), &MBB, MI, MBB.end());
   DoneMBB->transferSuccessors(&MBB);
   MBB.addSuccessor(LoopHeadMBB);
@@ -427,7 +430,10 @@ bool LoongArchExpandAtomicPseudo::expandAtomicMinMaxOp(
   BuildMI(LoopTailMBB, DL, TII->get(LoongArch::BEQZ))
       .addReg(Scratch1Reg)
       .addMBB(LoopHeadMBB);
-  BuildMI(LoopTailMBB, DL, TII->get(LoongArch::DBAR)).addImm(0x700);
+
+  // .tail:
+  //   dbar 0x700
+  BuildMI(TailMBB, DL, TII->get(LoongArch::DBAR)).addImm(0x700);
 
   NextMBBI = MBB.end();
   MI.eraseFromParent();
@@ -436,6 +442,7 @@ bool LoongArchExpandAtomicPseudo::expandAtomicMinMaxOp(
   computeAndAddLiveIns(LiveRegs, *LoopHeadMBB);
   computeAndAddLiveIns(LiveRegs, *LoopIfBodyMBB);
   computeAndAddLiveIns(LiveRegs, *LoopTailMBB);
+  computeAndAddLiveIns(LiveRegs, *TailMBB);
   computeAndAddLiveIns(LiveRegs, *DoneMBB);
 
   return true;

llvm/test/CodeGen/LoongArch/ir-instruction/atomicrmw-minmax.ll

@@ -1,5 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc --mtriple=loongarch64 < %s | FileCheck %s --check-prefix=LA64
+; RUN: llc --mtriple=loongarch64 --verify-machineinstrs < %s | \
+; RUN:   FileCheck %s --check-prefix=LA64
 
 ;; TODO: Testing for LA32 architecture will be added later
 
@@ -28,8 +29,9 @@ define i8 @atomicrmw_umax_i8_acquire(ptr %a, i8 %b) nounwind {
 ; LA64-NEXT:  .LBB0_3: # in Loop: Header=BB0_1 Depth=1
 ; LA64-NEXT:    sc.w $a5, $a2, 0
 ; LA64-NEXT:    beqz $a5, .LBB0_1
-; LA64-NEXT:    dbar 1792
 ; LA64-NEXT:  # %bb.4:
+; LA64-NEXT:    dbar 1792
+; LA64-NEXT:  # %bb.5:
 ; LA64-NEXT:    srl.w $a0, $a4, $a0
 ; LA64-NEXT:    ret
   %1 = atomicrmw umax ptr %a, i8 %b acquire
@@ -62,8 +64,9 @@ define i16 @atomicrmw_umax_i16_acquire(ptr %a, i16 %b) nounwind {
 ; LA64-NEXT:  .LBB1_3: # in Loop: Header=BB1_1 Depth=1
 ; LA64-NEXT:    sc.w $a5, $a2, 0
 ; LA64-NEXT:    beqz $a5, .LBB1_1
-; LA64-NEXT:    dbar 1792
 ; LA64-NEXT:  # %bb.4:
+; LA64-NEXT:    dbar 1792
+; LA64-NEXT:  # %bb.5:
 ; LA64-NEXT:    srl.w $a0, $a4, $a0
 ; LA64-NEXT:    ret
   %1 = atomicrmw umax ptr %a, i16 %b acquire
@@ -115,8 +118,9 @@ define i8 @atomicrmw_umin_i8_acquire(ptr %a, i8 %b) nounwind {
 ; LA64-NEXT:  .LBB4_3: # in Loop: Header=BB4_1 Depth=1
 ; LA64-NEXT:    sc.w $a5, $a2, 0
 ; LA64-NEXT:    beqz $a5, .LBB4_1
-; LA64-NEXT:    dbar 1792
 ; LA64-NEXT:  # %bb.4:
+; LA64-NEXT:    dbar 1792
+; LA64-NEXT:  # %bb.5:
 ; LA64-NEXT:    srl.w $a0, $a4, $a0
 ; LA64-NEXT:    ret
   %1 = atomicrmw umin ptr %a, i8 %b acquire
@@ -149,8 +153,9 @@ define i16 @atomicrmw_umin_i16_acquire(ptr %a, i16 %b) nounwind {
 ; LA64-NEXT:  .LBB5_3: # in Loop: Header=BB5_1 Depth=1
 ; LA64-NEXT:    sc.w $a5, $a2, 0
 ; LA64-NEXT:    beqz $a5, .LBB5_1
-; LA64-NEXT:    dbar 1792
 ; LA64-NEXT:  # %bb.4:
+; LA64-NEXT:    dbar 1792
+; LA64-NEXT:  # %bb.5:
 ; LA64-NEXT:    srl.w $a0, $a4, $a0
 ; LA64-NEXT:    ret
   %1 = atomicrmw umin ptr %a, i16 %b acquire