[AMDGPU] Update LiveVariables in convertToThreeAddress()
This can fix an asan failure like below.
==15856==ERROR: AddressSanitizer: use-after-poison on address ...
READ of size 8 at 0x6210001a3cb0 thread T0
#0 llvm::MachineInstr::getParent()
#1 llvm::LiveVariables::VarInfo::findKill()
#2 TwoAddressInstructionPass::rescheduleMIBelowKill()
#3 TwoAddressInstructionPass::tryInstructionTransform()
#4 TwoAddressInstructionPass::runOnMachineFunction()
We need to update the Kills if we replace instructions. The Kills
may be later accessed within TwoAddressInstruction pass.
Differential Revision: https://reviews.llvm.org/D89092