nir/loop_analyze: Bail if we encounter swizzles

None of the current code knows what to do with swizzles.  Take the safe
option for now and bail if we see one.  This does have a small shader-db
impact but it is at least safe.

Shader-db results on Kaby Lake:

    total loops in shared programs: 4364 -> 4388 (0.55%)
    loops in affected programs: 5 -> 29 (480.00%)
    helped: 5
    HURT: 29

Shader-db results on Haswell:

    total loops in shared programs: 4373 -> 4370 (-0.07%)
    loops in affected programs: 5 -> 2 (-60.00%)
    helped: 5
    HURT: 2

Fixes: 6772a17a "nir: Add a loop analysis pass"
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit 9a3cb6f5)