i965/vs: Keep track of indices into a per-register array for virtual GRFs.

Tracking virtual GRFs has tension between using a packed array per
virtual GRF (which is good for register allocation), and sparse arrays
where there's an element per actual register (so the first and second
column of a mat2 can be distinguished inside of an optimization pass).

The FS mostly avoided the need for this second sparse array by doing
virtual GRF splitting, but that meant that instances where virtual GRF
splitting didn't work, instructions using those registers got much
less optimized.

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