More identity folding for LOGAND and LOGIOR with constants

More identity folding for LOGAND and LOGIOR with constants

 * Handle more complex cases than only powers of two:
   compare the variant argument with a power-of-two-sized
   prefix of the constant bit pattern.

 * Add parallel logic for LOGIOR: if all the ones we're ORing in
   are already set because the variant argument is a small enough
   negative integer, we've got an identity.

 * This is a bit hairy, so exhaustively check the logic with small
   values.