liveness: do reverse walk during analysis phase and store results

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on February 28, 2012 14:08:08_

xref issue #777 now that we're using drmgr and splitting into formal phases, we should do our analysis just once which will not only fit in better w/ drmgr's recommendations but also be more performant:

store in bit array in new note fields

currently doing liveness at top of bb to pick scratch reg.
then doing forward liveness walk in fastpath_pre_instrument() and in
fastpath_pre_app_instr()! shouldn't those 2 have exact same results?

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=808_