Copy propagation on hard registers for the GNU compiler.
Copyright (C) 2000-2013 Free Software Foundation, Inc.
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The following code does forward propagation of hard register copies.
The object is to eliminate as many dependencies as possible, so that
we have the most scheduling freedom. As a side effect, we also clean
up some silly register allocation decisions made by reload. This
code may be obsoleted by a new register allocator.
DEBUG_INSNs aren't changed right away, as doing so might extend the
lifetime of a register and get the DEBUG_INSN subsequently reset.
So they are queued instead, and updated only when the register is
used in some subsequent real insn before it is set.
