i965: Add a lowering pass to convert TXD to TXL by computing the LOD.

Intel hardware doesn't natively support textureGrad with shadow
comparisons.  So we need to generate code to handle it somehow.

Based on the equations of page 205 of the OpenGL 3.0 specification,
it's possible to compute the LOD value that would be selected given the
gradient values.  Then, we can simply convert the TXD to a TXL.

Currently, this passes 34/46 of oglconform's shadow-grad subtests;
four cubemap tests are regressed.  We should investigate this in the
future.

v2: Apply abs() to the scalar case (thanks to Eric).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Eric Anholt <[email protected]>

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