Remove mterp reliance on code item layout

Pass dex instruction pointer intead of code item pointer to the mterp
entry code. This removes a dependency on the code item layout since it
may change in the future for compact dex.

Bug: 63756964
Test: test/testrunner/testrunner.py --host -j40

Change-Id: Icfffb2e17372439f0833ecce1c0ddb05e7e7e69c