i965/vec4: Use nir_move_vec_src_uses_to_dest

i965/vec4: Use nir_move_vec_src_uses_to_dest

The idea here is not that it gives register coalescing a little bit of a
helping hand.  It doesn't actually fix the coalescing problems, but it
seems to help a good bit.

Shader-db results for vec4 programs on Haswell:

   total instructions in shared programs: 1746280 -> 1683959 (-3.57%)
   instructions in affected programs:     1259166 -> 1196845 (-4.95%)
   helped:                                11363
   HURT:                                  148

v2 (Jason Ekstrand):
 - Run nir_move_vec_src_uses_to_dest after going out of SSA
 - New shader-db numbers

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>