[MachineSinking] Clear kill flag of all operands at all their uses.

When sinking an instruction it might be moved past the original last use of one
of its operands. This last use has the kill flag set and the verifier will
obviously complain about this.

Before Machine Sinking (AArch64):
%vreg3<def> = ASRVXr %vreg1, %vreg2<kill>
%XZR<def> = SUBSXrs %vreg4, %vreg1<kill>, 160, %NZCV<imp-def>
...

After Machine Sinking:
%XZR<def> = SUBSXrs %vreg4, %vreg1<kill>, 160, %NZCV<imp-def>
...
%vreg3<def> = ASRVXr %vreg1, %vreg2<kill>

This fix clears all the kill flags in all instruction that use the same operands
as the instruction that is being sunk.

This fixes rdar://problem/18180996.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216803 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineSink.cpp

@@ -736,9 +736,19 @@ bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
                          ++MachineBasicBlock::iterator(DbgMI));
   }
 
-  // Conservatively, clear any kill flags, since it's possible that they are no
-  // longer correct.
-  MI->clearKillInfo();
+  // When sinking the instruction the live time of its operands can be extended
+  // bejond their original last use (marked with a kill flag). Conservatively
+  // clear the kill flag in all instructions that use the same operand
+  // registers.
+  for (auto &MO : MI->uses())
+    if (MO.isReg() && MO.isUse()) {
+      // Preserve the kill flag for this instruction.
+      bool IsKill = MO.isKill();
+      // Clear the kill flag in all instruction that use this operand.
+      MRI->clearKillFlags(MO.getReg());
+      // Restore the kill flag for only this instruction.
+      MO.setIsKill(IsKill);
+    }
 
   return true;
 }