[TwoAddressInstruction] Fix stale LiveVariables info in processStatepoint

[foad]

Summary

D129213 improves verification of LiveVariables, and caused
CodeGen/X86/statepoint-cmp-sunk-past-statepoint.ll to fail with:

• Bad machine code: LiveVariables: Block should not be in AliveBlocks ***

after Two-Address instruction pass.

Fix it by clearing AliveBlocks for a register which is no longer used.

[dantrushin]

LGTM.
(We perhaps should have removed RegA completely from MRI, LiveVariables etc, but I did not find interface for this. Are Registers append-only?)

llvm/lib/CodeGen/TwoAddressInstructionPass.cpp

for (auto &TO : TiedOperands) {
     }
 
     if (LV) {
       if (MI->getOperand(SrcIdx).isKill())
         LV->removeVirtualRegisterKilled(RegB, *MI);
       LiveVariables::VarInfo &SrcInfo = LV->getVarInfo(RegB);
       LiveVariables::VarInfo &DstInfo = LV->getVarInfo(RegA);
       SrcInfo.AliveBlocks |= DstInfo.AliveBlocks;
+      DstInfo.AliveBlocks.clear();
       for (auto *KillMI : DstInfo.Kills)
         LV->addVirtualRegisterKilled(RegB, *KillMI, false);
     }
   }
   return !NeedCopy;
 }
 
 /// Reduce two-address instructions to two operands.