nir/gcm: dont move movs unless we can replace them later with their src

nir/gcm: dont move movs unless we can replace them later with their src

This helps us avoid moving the movs outside if branches when there
src can't be scalarized.

For example it avoids:

   vec4 32 ssa_7 = tex ssa_6 (coord), 0 (texture), 0 (sampler),
   if ... {
      r0 = imov ssa_7.z
      r1 = imov ssa_7.y
      r2 = imov ssa_7.x
      r3 = imov ssa_7.w
      ...
   } else {
      ...
      if ... {
         r0 = imov ssa_7.x
         r1 = imov ssa_7.w
         ...
      else {
         r0 = imov ssa_7.z
         r1 = imov ssa_7.y
         ...
      }
      r2 = imov ssa_7.x
      r3 = imov ssa_7.w
   }
   ...
   vec4 32 ssa_36 = vec4 r0, r1, r2, r3

Becoming something like:

   vec4 32 ssa_7 = tex ssa_6 (coord), 0 (texture), 0 (sampler),
   r0 = imov ssa_7.z
   r1 = imov ssa_7.y
   r2 = imov ssa_7.x
   r3 = imov ssa_7.w

   if ... {
      ...
   } else {
      if ... {
         r0 = imov r2
         r1 = imov r3
         ...
      else {
         ...
      }
      ...
   }

While this is has a smaller instruction count it requires more work
for the same result. With more complex examples we can also end up
shuffling the registers around in a way that requires more registers
to use as temps so that we don't overwrite our original values along
the way.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4636>