i965: Apply VS attribute workarounds in NIR.

This patch re-implements the pre-Haswell VS attribute workarounds.
Instead of emitting shader code in the vec4 backend, we now simply
call a NIR pass to emit the necessary code.

This simplifies the vec4 backend.  Beyond deleting code, it removes
the primary use of ATTR as a destination.  It also eliminates the
requirement that the vec4 VS backend express the ATTR file in terms
of VERT_ATTRIB_* locations, giving us a bit more flexibility.

This approach is a little different: rather than munging the attributes
at the top, we emit code to fix them up when they're accessed.  However,
we run the optimizer afterwards, so CSE should eliminate the redundant
math.  It may even be able to fuse it with other calculations based on
the input value.

shader-db does not handle non-default NOS settings, so I have no
statistics about this patch.

Note that the scalar backend does not implement VS attribute
workarounds, as they are unnecessary on hardware which allows SIMD8 VS.

v2: Do one multiply for FIXED rescaling and select components from
    either the original or scaled copy, rather than multiplying each
    component separately (suggested by Matt Turner).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Chris Forbes <[email protected]>

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