tests/sksl/runtime/LargeProgram_ZeroIterFor.skrp

    1. store_src_rg                   xy = src.rg
    2. init_lane_masks                CondMask = LoopMask = RetMask = true
    3. zero_slot_unmasked             i = 0
    4. copy_slot_unmasked             $0 = i
    5. cast_to_float_from_int         $0 = IntToFloat($0)
    6. swizzle_4                      $0..3 = ($0..3).xxxx
    7. load_src                       src.rgba = $0..3