nir/loop_analyze: Properly handle swizzles in loop conditions

This commit re-plumbs all of nir_loop_analyze to use nir_ssa_scalar for
all intermediate values so that we can properly handle swizzles.  Even
though if conditions are required to be scalars, they may still consume
swizzles so you could have ((a.yzw < b.zzx).xz && c.xx).y == 0 as your
loop termination condition.  The old code would just bail the moment it
saw its first non-zero swizzle but we can now properly chase the scalar
from the if condition to all the way to a, b, and c.

Shader-db results on Kaby Lake:

    total loops in shared programs: 4388 -> 4364 (-0.55%)
    loops in affected programs: 29 -> 5 (-82.76%)
    helped: 29
    HURT: 5

Shader-db results on Haswell:

    total loops in shared programs: 4370 -> 4373 (0.07%)
    loops in affected programs: 2 -> 5 (150.00%)
    helped: 2
    HURT: 5

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit ff972c7a)