nir/lower_vec_to_movs: Coalesce into destinations of fdot instructions

nir/lower_vec_to_movs: Coalesce into destinations of fdot instructions

Now that we have a replicating fdot instruction, we can actually coalesce
into the destinations of vec4 instructions.  We couldn't really do this
before because, if the destination had to end up in .z, we couldn't
reswizzle the instruction.  With a replicated destination, the result ends
up in all channels so we can just set the writemask and we're done.

Shader-db results for vec4 programs on Haswell:

   total instructions in shared programs: 1747753 -> 1746280 (-0.08%)
   instructions in affected programs:     143274 -> 141801 (-1.03%)
   helped:                                667
   HURT:                                  0

It turns out that dot-products matter...

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