Various debug ART GC performance improvements

Removed unnecessary read barriers in SizeOf and a few other places.
Disabled disallow read barrier check.

Before:
GC time: 15.817s
Real 0m26.113s
user  1m16.780s
sys 0m3.152s

After:
GC time: 9.212s
real  0m19.875s
user  1m9.916s
sys 0m1.916s

Bug: 35644369
Test: test-art-host

Change-Id: I79a65259deff2a478a96e02ae69b14730b6dcbe6