disas: nanoMIPS: Name some functions in a more descriptive way

Rename some functions that have names that are hard to understand.

Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Reviewed-by: Stefan Markovic <smarkovic@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>