do not copy DEBUG_EXPRs in copy_insn

[Alexandre Oliva]

copy_rtx avoids duplicating DEBUG_EXPRs, so that they map one-to-one to
DEBUG_EXPR_DECLs, but copy_insn lacks these smarts.  This was exposed by
a patch I'm working on, that uses RTL flags in DEBUG_EXPRs to speed up
expansion of var location notes within var-tracking.

Regstrapped on i686-pc-linux-gnu and x86_64-linux-gnu.  Ok to install?

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-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer