Allow the above combination of experiments to work together
correctly, fixing an encode/decode mismatch bug when they
were all enabled.

This change causes build_masked_compound(_highbd) to only
ever be called if CONFIG_SUPERTX is off, so wrap these functions
in an '#if !CONFIG_SUPERTX' block.

BUG=aomedia:313

Change-Id: Ic3886bc69ba9624b8fcb0a4c2d71fc64d2c0f22c