[RISCV][Peephole] Clear kill flags for registers after foldUndefPassthruVMV_V_V (#138847)

Without clearing kill flags, this pass will generate bad machine code.

```
*** Bad machine code: Using a killed virtual register ***
- function:    main
- basic block: %bb.0 entry (0x437ef928)
- instruction: %12:vrn7m1 = INSERT_SUBREG %11:vrn7m1(tied-def 0), %0:vr, %subreg.sub_vrm1_0
- operand 2:   %0:vr
```

Author: BeMg

llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp

@@ -584,6 +584,7 @@ bool RISCVVectorPeephole::foldUndefPassthruVMV_V_V(MachineInstr &MI) {
   }
 
   MRI->replaceRegWith(MI.getOperand(0).getReg(), MI.getOperand(2).getReg());
+  MRI->clearKillFlags(MI.getOperand(2).getReg());
   MI.eraseFromParent();
   return true;
 }

llvm/test/CodeGen/RISCV/rvv/vmv.v.v-peephole.mir

@@ -89,3 +89,19 @@ body: |
     %passthru:vr = COPY $v8
     %x:vr = PseudoVADD_VV_MF4 %passthru, $noreg, $noreg, 4, 4 /* e16 */, 0 /* tu, mu */
     %y:vr = PseudoVMV_V_V_MF8 %passthru, %x, 4, 3 /* e8 */, 0 /* tu, mu */
+...
+---
+name: kill_flag
+tracksRegLiveness: true
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: kill_flag
+    ; CHECK: [[PseudoVADD_VV_M1_:%[0-9]+]]:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 4 /* e16 */, 0 /* tu, mu */
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:vr = COPY [[PseudoVADD_VV_M1_]]
+    ; CHECK-NEXT: [[COPY1:%[0-9]+]]:vr = COPY [[PseudoVADD_VV_M1_]]
+    %0:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 4 /* e16 */, 0 /* tu, mu */
+    %1:vr = PseudoVMV_V_V_M1 $noreg, %0, -1, 4 /* e16 */, 0 /* tu, mu */
+    %2:vr = COPY killed %1
+    %3:vr = COPY %0
+...
+---