nir: add a new v3d-specific intrinsic for tile buffer color reads

nir: add a new v3d-specific intrinsic for tile buffer color reads

This is intended to be used, for example, with OpenGL logic operations. It
takes a render target as source and a sample index in the base index for
MSAA color reads.

v2: drop the CAN_ELIMINATE and CAN_REORDER flags (Eric).

Reviewed-by: Eric Anholt <eric@anholt.net>