[otbn] Refactor decode to merge alu and non alu control decode

The OTBN decoder was derived from the Ibex decoder which split the incoming instruction into two separate sets of flops for timing reasons. This is not required in OTBN. We should merge all of the decode back into a single case statement.

Start by locating the OTBN decoder and reading how its ALU and non-ALU control decode are currently split. Trace the incoming instruction through both decode paths, then verify that the refactor leaves one merged case statement with equivalent control outputs.

Written by the indexing model from the issue text.