i965: Use a separate register for every access to an SSA undef.

Previously, we allocated a new VGRF for every undefined definition.
Instead, this patch makes us allocate a new VGRF for every use of an
undefined definition.  This makes sure that undefined values are
fully independent of one another, and have live ranges limited to
their single use.  This allows register coalescing to combine the
source and destination of MOVs from undefined sources, eliminating
the MOV altogether.

On Broadwell:

total instructions in shared programs: 11641187 -> 11640214 (-0.01%)
instructions in affected programs: 70199 -> 69226 (-1.39%)
helped: 213
HURT: 1

v2: Add a comment (based on Iago's suggested one).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Iago Toral Quiroga <[email protected]>

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