cappuccino/wb_mux: remove unused lsu_valid_i input
execute_alu: remove & {OPTION_OPERAND_WIDTH{1'b1}} from mul

Several synthesis tools seems to be confused by the intention of this,
there have been reports about problems with Synplify and
ISE seem to add redundant logic.
execute_alu: Set result of disabled units to 0

Apart from being more sensible, it seems some tools can make
better optimization decisions from signals tied to 0.