Use ScopedArenaAllocator for Phi elimination pass.

Memory needed to compile the two most expensive methods for
aosp_angler-userdebug boot image:
  BatteryStats.dumpCheckinLocked() : 20.2MiB -> 20.0MiB (-195KiB)
  BatteryStats.dumpLocked(): 40.3MiB -> 39.9MiB (-458KiB)

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 64312607
Change-Id: Ie79b46cd42df476c2f66c6f869b1dd810ff15963