Remove duplicated code in av1_make_masked_inter_predictor

This patch basically just pushes #if CONFIG_HIGHBITDEPTH down past
other #if/#endif blocks, letting us pull out common code instead of
having to repeat it. There should be no functional change (indeed,
there should be no change except whitespace after preprocessing!)

Change-Id: I9bcbfbdec5ad54e42095de085df381e85bf48197