Don't use MuxLookup default for full mapping

This modifies MuxLookup to not use the 'default' mapping argument if a
"full" mapping is provided. A "full" mapping enumerates all possible
cases for a 'key' argument of a known size. This will check literal
values to ensure exhaustiveness holds.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: Albert Magyar <albert.magyar@gmail.com>