[vta] handle auto-inc-dec in debug insns in combine

[Alexandre Oliva]

When combine substitutes an auto-inc-dec in a set into a use, and
there's a debug insn that references the set pseudo, we lose (*)
unless we adjust the addressing mode to lose the auto-inc-dec.  This
is what the patch below accomplishes, fixing problems in ia64.

(*) say, debug insns suddently become setters, oops

While at that, I realized that propagate_for_debug was too
conservative in moving or marking for rescan debug insns that it found
between the combined insns.  Marking insns for rescan without changes
is "just" wasteful, but needlessly moving debug insns around, that are
supposed to mark assignment points, is detrimental to the quality of
debug information, so we must only move them if there's absolutely no
way around it.

I'm installing this in the vta branch.

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-- 
Alexandre Oliva         http://www.lsd.ic.unicamp.br/~oliva/
FSF Latin America Board Member         http://www.fsfla.org/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}