Move reg_equiv* arrays into a single VEC structure

[Jeff Law]

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As originally discussed here:

http://gcc.gnu.org/ml/gcc-patches/2010-05/msg00987.html

Changes since the original submission:

Per Richi's suggestion I changed the patch to use a GC'd VEC.  I defined
accessor macros so that a ton of reformatting isn't needed and finally I
made the appropriate tweaks to the few backends that peeked at the
reg_equiv arrays.

Given the changes and the length of time since the original submission,
I think it's probably best to get a fresh approval rather than rely on
the prior approval.

Bootstrapped and regression tested on x86_64-unknown-linux-gnu.

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