[RISCV] Fix a bug where AVL is the last MI in MBB. (llvm#144668)

When `AVL` is the last MI, `std::next(II)` equals `MBB.end()`, and
calling `II->getParent()` at that point will cause an error.

---------

Co-authored-by: yanming <ming.yan@terapines.com>

Author: NexMing

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -1119,25 +1119,26 @@ void RISCVInsertVSETVLI::insertVSETVLI(MachineBasicBlock &MBB,
     LIS->InsertMachineInstrInMaps(*MI);
     LiveInterval &LI = LIS->getInterval(AVLReg);
     SlotIndex SI = LIS->getInstructionIndex(*MI).getRegSlot();
+    const VNInfo *CurVNI = Info.getAVLVNInfo();
     // If the AVL value isn't live at MI, do a quick check to see if it's easily
     // extendable. Otherwise, we need to copy it.
-    if (LI.getVNInfoBefore(SI) != Info.getAVLVNInfo()) {
+    if (LI.getVNInfoBefore(SI) != CurVNI) {
       if (!LI.liveAt(SI) && LI.containsOneValue())
         LIS->extendToIndices(LI, SI);
       else {
         Register AVLCopyReg =
             MRI->createVirtualRegister(&RISCV::GPRNoX0RegClass);
+        MachineBasicBlock *MBB = LIS->getMBBFromIndex(CurVNI->def);
         MachineBasicBlock::iterator II;
-        if (Info.getAVLVNInfo()->isPHIDef())
-          II = LIS->getMBBFromIndex(Info.getAVLVNInfo()->def)->getFirstNonPHI();
+        if (CurVNI->isPHIDef())
+          II = MBB->getFirstNonPHI();
         else {
-          II = LIS->getInstructionFromIndex(Info.getAVLVNInfo()->def);
+          II = LIS->getInstructionFromIndex(CurVNI->def);
           II = std::next(II);
         }
         assert(II.isValid());
-        auto AVLCopy =
-            BuildMI(*II->getParent(), II, DL, TII->get(RISCV::COPY), AVLCopyReg)
-                .addReg(AVLReg);
+        auto AVLCopy = BuildMI(*MBB, II, DL, TII->get(RISCV::COPY), AVLCopyReg)
+                           .addReg(AVLReg);
         LIS->InsertMachineInstrInMaps(*AVLCopy);
         MI->getOperand(1).setReg(AVLCopyReg);
         LIS->createAndComputeVirtRegInterval(AVLCopyReg);

llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.mir

@@ -142,6 +142,10 @@
     ret void
   }
 
+  define void @avl_is_last_instr() {
+    ret void
+  }
+
   declare i32 @llvm.vector.reduce.add.v4i32(<4 x i32>)
 
   declare <vscale x 1 x i64> @llvm.riscv.vadd.nxv1i64.nxv1i64.i64(<vscale x 1 x i64>, <vscale x 1 x i64>, <vscale x 1 x i64>, i64) #1
@@ -1099,3 +1103,31 @@ body: |
     renamable $v10m2 = PseudoVADD_VV_M2 undef renamable $v10m2, %v, %v, -1, 5, 0
     renamable $v8m2 = PseudoVADD_VV_M2 undef renamable $v8m2, killed renamable $v10m2, killed %v, %outvl:gprnox0, 5, 0
     PseudoRET implicit $v8m2
+...
+---
+name: avl_is_last_instr
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: avl_is_last_instr
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $x10
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %avl:gprnox0 = COPY $x10
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:gprnox0 = COPY %avl
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   dead %avl:gprnox0 = ADDI %avl, -1
+  ; CHECK-NEXT:   dead $x0 = PseudoVSETIVLI 1, 192 /* e8, m1, ta, ma */, implicit-def $vl, implicit-def $vtype
+  ; CHECK-NEXT:   $v8 = PseudoVMV_S_X undef renamable $v8, $x0, 1, 3 /* e8 */, implicit $vl, implicit $vtype
+  ; CHECK-NEXT:   dead $x0 = PseudoVSETVLI [[COPY]], 192 /* e8, m1, ta, ma */, implicit-def $vl, implicit-def $vtype
+  ; CHECK-NEXT:   $v8 = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, $noreg, 3 /* e8 */, 3 /* ta, ma */, implicit $vl, implicit $vtype
+  bb.0:
+    liveins: $x10
+    %avl:gprnox0 = COPY $x10
+
+  bb.1:
+    %vl:gprnox0 = PseudoVSETVLI %avl:gprnox0, 192, implicit-def dead $vl, implicit-def dead $vtype
+    %avl:gprnox0 = ADDI %avl:gprnox0, -1
+    $v8 = PseudoVMV_S_X undef renamable $v8, $x0, 1, 3
+    $v8 = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, %vl:gprnox0, 3, 3