Avoid unnecessary pred calculation in compound_type_rd
Avoid unnecessary pred calculation in compound_type_rd

Calculate the pred for masked compound case only when
the function build_and_cost_compound_type is called.
By doing so unnecessary pred calculation in the function
compound_type_rd is avoided.

Change-Id: Idcfb8783f2b4567903b65a86586e8d64e2109a51

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom