ipa/127479 - preserve PHI operand order with LTO streaming

LTO streaming does not preserve the order of incoming edges and
thus the order of PHI operands, disturbing the IPA ICF IPA reference
comparison.  The following rectifies this by streaming predecessor
edges instead of successor edges (which leaves outgoing edge order
not preserved)

	PR ipa/127479
	* lto-streamer-out.cc (output_cfg): Output predecessor edges.
	* lto-streamer-in.cc (input_cfg): Input predecessor edges.

	* gcc.dg/lto/pr127479_0.c: New testcase.