iris: Enable nir_opt_large_constants

iris: Enable nir_opt_large_constants

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15306230 -> 15304726 (<.01%)
    instructions in affected programs: 4570 -> 3066 (-32.91%)
    helped: 16
    HURT: 0

    total cycles in shared programs: 361703436 -> 361680041 (<.01%)
    cycles in affected programs: 129388 -> 105993 (-18.08%)
    helped: 16
    HURT: 0

    LOST:   0
    GAINED: 2

The helped programs were in XCom 2, Deus Ex: Mankind Divided, and Kerbal
Space Program

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