[optimizing compiler] Rename dex_offset to dex_pc in the builder

The term dex offset comes from Dalvik where these were addresses in
memory. Currently these should be dex PCs, namely offsets from the start
of code items. Quick lacks this clean up but there's no need to bring
over the convention to optimizing.

(thanks to irogers for the commit message text)

Change-Id: If86c0cf021a06f04a21462053e9d5f31bba7f65d