Well, whadayaknow... Tom Tromey pointed me at --regex which we can use
to add patterns for not only the .opt files, but for a bunch of other
files/languages we define in GCC.
The following patch adds support for common.opt, rtl.def, tree.def, and
gimple.def. Now you can use your editor to tag things like GIMPLE_NOP,
and PLUS_EXPR, which means I'll get lost less often.
Kinda neat, IMO.
Tested by inspecting TAGS.sub manually, as well as searching for random
stuff with both vi and emacs.
OK for mainline?