ART: Mterp read barrier fix + minor cleanup

Read barrier support relies on hooks in common code for loading
object references.  Mterp missed doing this for iget-object-quick.
Also, added missing conditional assembly around debug event logging
for mterp fallback and deleted an unnecessary store.

Bug: 26510411

Change-Id: I2d5b27c4090be58d3cfcb14309d14ccabf04a6f5