Patch to improve PIC find_base_term

[John Wehle]

> In fact, we already get REG_EQUAL notes for this on at least a
> few platforms.  Are you sure they're not there for x86?

In fact, they are there (I have yet to figure out who's adding them). :-)

What I'm not sure (now that I see the notes) is whether alias analysis
is making the best use of them (i.e. canon_rtx isn't always used).  I
made some "play" changes and got better results, however I need to poke
around some more regarding this and the GOTOFF alias analysis issues.

-- John
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