Add couple DexCache read-barriers.

I intend to make some of the fields mutable in the next CL,
which means we cannot skip the read-barrier any more.

Split this into own CL to make it easier to check if
there are any significant performance changes.

Bug: 182072259
Test: test.py -b -r --host --64
Change-Id: Ia20a3ba8285eb56ae6ff9e7fa5c0a62b14d88b65
1 file changed