Improve stack walk performance.

Move a few functions from art_method.cc to art_method-inl.h
and introduce new overloads that take already known partial
results; do not rely on the compiler to magically merge the
identical but non-trivial paths. The partial results are
DCHECKed to be correct.

Change-Id: I342c3001bbff08a2bbbb9a7b62ae67188ad8cffc