Combined with memset inlining, this has shown an improvement of over 11% on
the eon benchmark. This the only C++ program in spec2k and it seems C++
programs have a significantly larger number of memset/memcpy calls. Other
benchmarks also showed improvement of ~5% (perlbmk, parser) while most had
a 1-2% improvement.

This commit also includes a refactoring of lowerMemset which is much more
readable and also removed the fake use of the destination pointer register.

BUG=
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1279833005.