Fix opt --help ordering of available optimizations.

Introduced in -r283004, the PassNameParser sorts Optimization options in 
reverse. This is because the commit replaced a compare function with "<" 
(which would seemingly be proper based on the name of the comparison function). 
The result is the 'true' result is converted to '1', which is inverted.

This patch fixes this by replacing the '<' operator call on StringRef with a 
call to the StringRef compare function. It also renames the function to better 
reflect its meaning.

Differential Revision: https://reviews.llvm.org/D34831

llvm-svn: 306857