Shadow memory performance improvements.

This improves the various memory scanning helper functions in the shadow memory.
From the new perf tests that have been added to the RTL unittests:

MarkAsFreed
Before: 1502621466
After :  446056372 (3.37x faster)

ScanRightForBracketingBlockEnd
Before: 1501712122
After :   24537401 (62.5x faster)

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/175520043