Fix twolf -funroll-loops -O3 miscompilation (a semi-latent web.c bug)

[Dominique Dhumieres]

> Or rather this one. Same hammer, different color. It turns out that
> the rtlanal.c change caused problems, so I've got to use a home-brewn
> equivalent of remove_reg_equal_equiv_notes_for_regno...

This does not fix pr55006 on x86_64-apple-darwin10;-(while the patch in
http://gcc.gnu.org/ml/gcc-patches/2012-11/msg01607.html
does).

> Test case is unchanged so I'm omitting it here.

This test passes on x86_64-apple-darwin10 for a clean trunk.

Thanks for looking at the problem.

Dominique