[otbn] Save one datapath mux input in bignum ALU by leveraging blanking
[otbn] Save one datapath mux input in bignum ALU by leveraging blanking

Whenever logical_res is actually used,
alu_predec_bignum_i.logic_shifter_en is set to 1 and the blanking
disabled. Otherwise, logical_op_shifter_res_blanked is blanked to zero.
This can be leveraged to drive logical_res to 0 whenever it's not used.

Signed-off-by: Pirmin Vogel <386a7fc8e7338ed7899b4d9883c0ebec5de81366@lowrisc.org>

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