nir/lower_vec_to_movs: Coalesce movs on-the-fly when possible

nir/lower_vec_to_movs: Coalesce movs on-the-fly when possible

The old pass blindly inserted a bunch of moves into the shader with no
concern for whether or not it was really needed.  This adds code to try and
coalesce into the destination of the instruction providing the value.

Shader-db results for vec4 shaders on Haswell:

   total instructions in shared programs: 1754420 -> 1747753 (-0.38%)
   instructions in affected programs:     231230 -> 224563 (-2.88%)
   helped:                                1017
   HURT:                                  2

This approach is heavily based on a different patch by Eduardo Lima Mitev
<elima@igalia.com>.  Eduardo's patch did this in a separate pass as opposed
to integrating it into nir_lower_vec_to_movs.

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