New attempt: subword-based DCE, PR42575

[David Edelsohn]

This patch may have caused a new bootstrap ICE on PowerPC:

/farm/dje/src/src/gcc/cfg.c: In function 'scale_bbs_frequencies_gcov_type':
/farm/dje/src/src/gcc/cfg.c:1109:1: internal compiler error: in
delete_corresponding_reg_eq_notes, at dce.c:495

I see the error on powerpc-ibm-aix5.3 and I was told the error also
occurs on powerpc-darwin.

David