WIP: nir: add support to load_shared on Eric's prepass scheduling
That change allows the following 3.1 ES CTS test passing:
But the patch that introduces the nir scheduler introduces regressions on the following tests (so another reason for the WIP)
I plan to take a look to those.
Final note: in order to check that the load_shared is not moved if there are a store_shared between the original and the new position of the load_shared, I wrote this small test: piglit!126 (merged)