nir/lower_non_uniform: safely iterate over blocks

nir/lower_non_uniform: safely iterate over blocks

This fixes a problem where the same instruction gets replaced twice.
This was happening when the replaced instruction would be at the end
of a block.

Replacement of :

   if ssa_8 {
                ....
      intrinsic bindless_image_store (ssa_44, ssa_16, ssa_0, ssa_15) (5, 0, 34836, 32) /* image_dim=Buf */ /* image_array=false */ /* format=34836 */ /* access=32 */
   }

Would be :

   if ssa_8 {
      loop {
         vec1 32 ssa_47 = intrinsic read_first_invocation (ssa_44) ()
         vec1 1 ssa_48 = ieq ssa_47, ssa_44
         if ssa_48 {
            loop {
               vec1 32 ssa_49 = intrinsic read_first_invocation (ssa_44) ()
               vec1 1 ssa_50 = ieq ssa_49, ssa_44
               if ssa_50 {
                  intrinsic bindless_image_store (ssa_44, ssa_16, ssa_0, ssa_15) (5, 0, 34836, 32) /* image_dim=Buf */ /* image_array=false */ /* format=34836 */ /* access=32 */
                  break
               } else {
        ....
   }

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: 3bd545764151 ("nir: Add a lowering pass for non-uniform resource access")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>