Revert "ADT: SmallVector size/capacity use word-size integers when elements are small"

This reverts commit b8d08e96.

This change causes a 1% compile-time and 1% memory usage regression:

http://llvm-compile-time-tracker.com/compare.php?from=73b7dd1fb3c17a4ac4b1f1e603f26fa708009649&to=b8d08e961df1d229872c785ebdbc8367432e9752&stat=instructions
http://llvm-compile-time-tracker.com/compare.php?from=73b7dd1fb3c17a4ac4b1f1e603f26fa708009649&to=b8d08e961df1d229872c785ebdbc8367432e9752&stat=max-rss