[AArch64] Use MachineRegisterInfo instead of LiveIntervals to calculate liveness. NFC.

The CondOpt pass currently uses LiveIntervals to set the dead flag on a def.  This patch uses MachineRegisterInfo::use_empty instead as that is equivalent to the def being dead.

This removes an instance of LiveIntervals in the pass manager pipeline and saves 3.8% of compile time on llc conpiled for AArch64.

Reviewed by Chad Rosier and Zhaoshi.

llvm-svn: 235532