nterp/arm64: Clean up branch PC calculation.

Fuse two instructions into one. The new calculation is also
technically more correct, even though we should never see
branch offsets over 2^30 where the result would differ.

Test: testrunner.py --target --64 --interpreter
Bug: 112676029
Change-Id: Ib6db93ad90ee1b4bcb68839c64ded9dca101352e
1 file changed