i965: make use of nir_lower_returns() for GL

i965: make use of nir_lower_returns() for GL

Fixes two new piglit tests:

spec/glsl-1.10/execution/vs-nested-return-sibling-loop.shader_test
spec/glsl-1.10/execution/vs-nested-return-sibling-loop2.shader_test

shader-db results for BDW:

total instructions in shared programs: 12903158 -> 12903134 (-0.00%)
instructions in affected programs: 27100 -> 27076 (-0.09%)
helped: 32
HURT: 6

total cycles in shared programs: 294922518 -> 294922804 (0.00%)
cycles in affected programs: 4372828 -> 4373114 (0.01%)
helped: 31
HURT: 8

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>