Patch to improve PIC find_base_term

[John Wehle]

> I'm very surprised by this.  All of the memory loads from the GOT
> should already be marked as RTX_UNCHANGING_P, which should take 
> care of everything wrt aliasing.  What is slipping through?

The change to find_base_term wasn't primarily for memory loads from
the GOT ... the performance difference is probably due to other
aliasing issues addressed by the patch (which I'm re-examining /
reworking based on everyone's feedback).

I did apply the suggestion from Geoff regarding assigning an alias
set for loads from the GOT and there does seem to be a minor
performance difference.  Currently alias analysis assumes that
an unchanging write and an unchanging read may conflict, unless
there's information to the contrary so maybe that's what is
"slipping through" ... I don't actually know.  I've copied
you on the patch.

-- John
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