* Rename the 'masked_compound_*' functions to just 'masked_*'.
  The previous names were intended to be temporary, to distinguish
  the old and new masked motion search pipelines. But now that the
  old pipeline has been removed, we can reuse the old names.

* Simplify the new ext-inter compound motion search pipeline
  a bit.

* Harmonize names: Rename
  aom_highbd_masked_compound_sub_pixel_variance* to
  aom_highbd_8_masked_sub_pixel_variance*, to match the naming of
  the corresponding non-masked functions

Change-Id: I988768ffe2f42a942405b7d8e93a2757a012dca3