i965/fs: Simplify the register allocator using a map from RA reg to GRF.
i965/fs: Simplify the register allocator using a map from RA reg to GRF.

It's fewer pointers to track, and when we start caching the register
set, should be algorithmically better in the cache hit case (lookup in
a byte-per-register array, instead of a linear walk through
desctiption of register classes to find how to translate that class).

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