write_verilog: Lower $bwmux in expr mode

[povik]

What are the reasons/motivation for this change?

write_verilog doesn't know how to directly write an expression for $bwmux cells, so those are printed as an internall cell even if outside the -noexpr mode. To address, call bwmuxmap like we do to the same effect in btor/firrtl/smv backends.

Fixes #4751

[povik]

From the CI:

xprop_bwmux_1: mismatch
output differences:
  - sim sim_xprop vsim_expr_xprop vsim_noexpr vsim_noexpr_xprop sat sat_xprop opt_expr opt_expr_xprop
  - vsim_expr
make[1]: *** [run-test.mk:689: xprop_bwmux_1] Error 1
make[1]: *** Waiting for unfinished jobs....
xprop_pmux_3_1: ok
xprop_pmux_3_1: ok
xprop_bwmux_3: mismatch
output differences:
  - sim sim_xprop vsim_expr_xprop vsim_noexpr vsim_noexpr_xprop sat sat_xprop opt_expr opt_expr_xprop
  - vsim_expr
make[1]: *** [run-test.mk:696: xprop_bwmux_3] Error 1

The mapping by bwmuxmap isn't x-prop faithful