Split the structure TTEntry into hot (small) and cold (large) parts,

Split the structure TTEntry into hot (small) and cold (large) parts,
TTEntryH and TTEntryC.  TTEntryH is a VexGuestExtents plus one more
field.  For scenarios involving a lot of code discarding, when the
fast-path discard mechanism does not apply, this change reduces
significantly the number of LLC misses, because such discarding
involves sequentially searching the arrays of TTEntryH's.  For recent
Firefoxes the miss rate in a 6MB L3 cache is reduced by about 1/3, as
measured by /usr/bin/perf.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15874