* Implement optimisation in storev8/16/64 similar to  optimisations in storev32

See comments in MC_(helperc_STOREV8) for a detailed explanation.

On modern cpu, this gives a few percent speed improvement for
some perf tests, and reduces the nr of undistinguished secondary
map.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13904 a5019735-40e9-0310-863c-91ae7b9d1cf9