nir: add pass to move load_const

nir: add pass to move load_const

Run this pass late (after opt loop) to move load_const instructions back
into the basic blocks which use the result, in cases where a load_const
is only consumed in a single block.

This helps reduce register usage in cases where the backend driver
cannot lower the load_const to a uniform.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>