organization of optimization options in manual

[Sandra Loosemore]

The "Options That Control Optimization" section of the manual is
currently divided into three parts (not subsections, just separate 
option lists):

(1) General options like -O[n]

(2) Options that individually control options enabled by default at some 
-O[n] setting

(3) Options controlling optimizations that aren't enabled by default, or 
that are experimental

I've noted that a lot of options that belong in group (3) have been 
added to group (2), and at least one from (2) into group (3).  I'm 
thinking that the distinction between (2) and (3) is not really useful 
anyway; there are already both lists of which options are enabled at 
each -O level, and info in the descriptions of individual options to say 
what -O levels they're enabled at.

What would you think about reorganizing this section to add some 
subsections grouping options by purpose, instead?  E.g., loop 
optimizations, floating-point optimizations, inlining, LTO, profiling 
options, etc?  The section is almost 60 pages long in the printed manual 
and adding some structure would probably make it easier for users to 
find things.

The other option would be just to list the options alphabetically, but 
the index already does that if readers know the name of what they're 
looking for (or they can search for it in their browser).

-Sandra