I know we're deep into stage3 and about to transition to stage4.  So if 
the consensus is for this to wait, I'll understand....

This it the V3 of the ext-dce patch based on Joern's work from last year.

Changes since V2:
   Handle MINUS
   Minor logic cleanup for SUBREGs in ext_dce_process_sets
   Includes Joern's carry_backpropagate work
   Cleaned up and removed some use handling code for STRICT_LOW_PART
   Moved non-local goto special case out of main use handling, similar to
   how we handle CALL_INSN_FUSAGE
   Use df_simple_dataflow rather than custom dataflow handling

There's more cleanups we could be doing here, but the question is do we 
stop commit what we've got and iterate on the trunk or do we defer until 
gcc-15 in which case we iterate on a branch or something.



This still is enabled at -O1 or above, but that's to get as much testing 
as possible.  Assuming the rest is ACK'd for the trunk we'll put it into 
the list of optimizations enabled by -O2.