A new copy propagation and PHI elimination pass

This patch adds the strongly-connected copy propagation (SCCOPY) pass.
It is a lightweight GIMPLE copy propagation pass that also removes some
redundant PHI statements. It handles degenerate PHIs, e.g.:

_5 = PHI <_1>;
_6 = PHI <_6, _6, _1, _1>;
_7 = PHI <16, _7>;
// Replaces occurences of _5 and _6 by _1 and _7 by 16

It also handles more complicated situations, e.g.:

_8 = PHI <_9, _10>;
_9 = PHI <_8, _10>;
_10 = PHI <_8, _9, _1>;
// Replaces occurences of _8, _9 and _10 by _1

gcc/ChangeLog:

	* Makefile.in: Added sccopy pass.
	* passes.def: Added sccopy pass before LTO streaming and before
	RTL expansion.
	* tree-pass.h (make_pass_sccopy): Added sccopy pass.
	* gimple-ssa-sccopy.cc: New file.

gcc/testsuite/ChangeLog:

	* gcc.dg/sccopy-1.c: New test.

Signed-off-by: Filip Kastl <fkastl@suse.cz>

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