This is a workaround for the webgl2 conformance test case
WebglConformance_conformance2_glsl3_vector_dynamic_indexing_swizzled_lvalue.
Dynamic indexing of swizzled lvalue like "v.zyx[i] = 0.0" is problematic on
various platforms. This removes the indexing by translating it this way:

void dyn_index_write_vec3(inout vec3 base, in int index, in float value){
  switch (index) {
    case (0):
      (base[0] = value);
      return ;
    case (1):
      (base[1] = value);
      return ;
    case (2):
      (base[2] = value);
      return ;
    default:
      break;
  }
  if ((index < 0))
  {
    (base[0] = value);
    return ;
  }
  {
    (base[2] = value);
  }
}

...
dyn_index_write_vec3(v.zyx, i, 0.0);
...

Bug: chromium:709351
Change-Id: I971b38eb404209b56e6764af1063878c078a7e88
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1869109
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>