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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@235532 91177308-0d34-0410-b5e6-96231b3b80d8

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