mcpu: Automatically build the DISPATCH_TABLE

While reviewing dato's interdiff for the OpAssignGroupNodes, I
realised that we can do better. This patch replaces the hand-built
DISPATCH_TABLE with one built from the opcode.OP_MAPPING dict.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>