Explicitly set `alu_op_x_mux_sel` in decoder
Explicitly set `alu_op_x_mux_sel` in decoder

To make the regfile address check in RV32E not triggering false alarms
when accessing reg x16 - x31, the decoder should only set the muxes
for the regfile output when the instruction actually requires the
regfile output.

It is cleaner to explicitly set the `alu_op_x_mux_sel` signals to
`OP_X_REG_X` instead of relying on the default assignment if they
are really needed.

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