tests/sksl/shared/SwizzleLTRB.skrp

    1. store_src_rg                   coords = src.rg
    2. init_lane_masks                CondMask = LoopMask = RetMask = true
    3. copy_4_constants               $0..3 = colorRed
    4. swizzle_4                      $0..3 = ($0..3).zwyx
    5. copy_4_slots_unmasked          [main].result = $0..3
    6. load_src                       src.rgba = [main].result