How to determine the a use reference is the last reference of a virtual Register through current path

Hello llvmers:
      I'm study Fast register allocation in codegen. I get confused
with isKill flag in MachineOperand. I thin "isKill Because Fast Register
doesn't require LiveVariable Analysis , So during Fast RegisterAllocation,
the Allocation procedure can't determine if this use reference is the last
use ( isKill flag is missed for global Virtual register without pre-pass of
LiveVariable analysis );

So, How does Fast RegisterAllocation resolve such problem ?