INSN_SCOPE changes break mips-elf

[law@redhat.com]

mips-elf has had regressions ever since the INSN_SCOPE changes were installed
into the tree.  While the number of regressions has gone down since the
introduction of the INSN_SCOPE changes one regression still remains.

The remaining regression is gcc.dg/debug/debug-6 where we manage to lose
all debug information for "xyzzy".  I believe the fundamental problem is your
code in scope_to_insns_initialize does not properly handle SEQUENCEs as
generated by the delay slot filling pass.

While we do expect some lossage of debugging information when optimizing, this
case seems excessive and I think you should fix it.  It's probably also wise
to pay more attention to the regression tester after invasive changes such
as the INSN_SCOPE work.

[ I made a naive attempt to fix this, but it generated more problems than it
  solved. ]


Jeff