[RISCV] Clear kill flags for FalseReg in foldVMergeToMask

Or we can't pass the MachineVerifier because of using a killed virtual
register.

This was found when backporting llvm#170070 to 21.x branch.

Author: wangpc-pp

llvm/lib/Target/RISCV/RISCVVectorPeephole.cpp

@@ -834,6 +834,8 @@ bool RISCVVectorPeephole::foldVMergeToMask(MachineInstr &MI) const {
     MRI->constrainRegClass(
         MO.getReg(), True.getRegClassConstraint(MO.getOperandNo(), TII, TRI));
   }
+  // We should clear the IsKill flag since we have an use now.
+  MRI->clearKillFlags(FalseReg);
   MI.eraseFromParent();
 
   return true;

llvm/test/CodeGen/RISCV/rvv/vmerge-peephole.mir

@@ -148,7 +148,7 @@ body: |
     ; CHECK-NEXT: %y:vr = COPY $v9
     ; CHECK-NEXT: %mask:vmv0 = COPY $v0
     ; CHECK-NEXT: %add0:vr = PseudoVADD_VV_M1 $noreg, %x, %y, -1, 5 /* e32 */, 3 /* ta, ma */
-    ; CHECK-NEXT: %add1:vrnov0 = COPY killed %add:vrnov0
+    ; CHECK-NEXT: %add1:vrnov0 = COPY %add:vrnov0
     ; CHECK-NEXT: %merge:vrnov0 = PseudoVOR_VV_M1_MASK %add:vrnov0, %add1, %y, %mask, -1, 5 /* e32 */, 1 /* ta, mu */
     ; CHECK-NEXT: $v8 = COPY %merge
     ; CHECK-NEXT: PseudoRET implicit $v8