R600: Fix regression with shadow array sampler on pre-SI GPUs.

'R600/SI: Use proper instructions for array/shadow samplers.' removed two
cases from TEX_SHADOW. Vincent Lejeune reported on IRC that this broke some
shadow array piglit tests with the r600g driver. Reinstating the removed
cases should fix this, and still works with radeonsi as well.

I will follow up with some lit tests which would have caught the regression.

NOTE: This is a candidate for the Mesa stable branch.

Tested-by: Vincent Lejeune <vljn@ovi.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>