[Bug rtl-optimization/63620] RELOAD lost SET_GOT dependency on Darwin

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63620

--- Comment #12 from Jeffrey A. Law <law at redhat dot com> ---
The more I watch the %ebx PIC problems, the more this reminds me of secondary
reloads and I wonder if we defined those properly if the right things would
happen.

This kind of thing has shown up on other architectures.  For example, on the
PA, if you try to load a symbolic constant into an FP register while generating
PIC code, you have to generate a secondary reload using %r1 as a scratch.

It's not a perfect match in that we don't need a scratch, but instead the PIC
register to be live, but it's pretty damn close.

I haven't looked at LRA's handling of secondary reloads, but it may be worth
someone's time to do so to see if it can be used to show %ebx as being needed
in these cases.