abc9: fix SCC issues (#2694)

abc9: fix SCC issues (#2694)

* xilinx: add SCC test for DSP48E1

* xilinx: Gate DSP48E1 being a whitebox behind ALLOW_WHITEBOX_DSP48E1

Have a test that checks it works through ABC9 when enabled

* abc9 to break SCCs using $__ABC9_SCC_BREAKER module

* Add test

* abc9_ops: remove refs to (* abc9_keep *) on wires

* abc9_ops: do not bypass cells in an SCC

* Add myself to CODEOWNERS for abc9*

* Fix compile

* abc9_ops: run -prep_hier before scc

* Fix tests

* Remove bug reference pending fix

* abc9: fix for -prep_hier -dff

* xaiger: restore PI handling

* abc9_ops: -prep_xaiger sigmap

* abc9_ops: -mark_scc -> -break_scc

* abc9: eliminate hard-coded abc9.box from tests

Also tidy up

* Address review