Redo the dispatcher's fast-cache mechanism (VG_(tt_fast) et al) to be

Redo the dispatcher's fast-cache mechanism (VG_(tt_fast) et al) to be
more cache friendly.  This changes the mechanism from being a table of
pointers to (guest address, translated code pairs) to being a table of
pairs (guest address, pointer to translated code).  The effect ranges
from zero up to about 20% performance improvement on memcheck, the
biggest effects being seen for programs which jump around a large
number of blocks of code and whose data set does not fit in L2.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@6582