Further robustify the keyed lookup cache against unlucky hash

Further robustify the keyed lookup cache against unlucky hash
seeds.  This change is performance neutral on most snapshot
VM builds, but provides a big improvement on string-fasta
on around 5% of builds.
Committed: https://code.google.com/p/v8/source/detail?r=10478

[Yang, 2012-01-23 15:53:31]

Assuming that there is no generated code equivalent to KeyedLookupCache::Update,
this LGTM with one small comment.

https://chromiumcodereview.appspot.com/9193015/diff/1/src/heap.h
File src/heap.h (right):

https://chromiumcodereview.appspot.com/9193015/diff/1/src/heap.h#newcode2142
src/heap.h:2142: static const int kEntriesPerBucket = 4;
Maybe add assertions such as
STATIC_ASSERT(IS_POWER_OF_TWO(kEntriesPerBucket));
STATIC_ASSERT(kHashMask == -kEntriesPerBucket)