phiopt: let cond replication change the loop structure [PR127085]

Per Richi's suggestion in the PR, making an irreducible region
reducible is a good thing, so instead of preventing it just let loop
fixup discover the new loop: set LOOPS_NEED_FIXUP when we changed the
CFG, and drop preserves_loop_structure_p in favor of never duplicating
a loop header.

Tested on ppc64le Linux.

	PR tree-optimization/127085

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (preserves_loop_structure_p): Remove.
	(replicate_cond_into_pred): Do not call it.
	(replicate_conds_over_phis): Never duplicate loop headers; do
	not mark back edges; set LOOPS_NEED_FIXUP when the CFG changed.

gcc/testsuite/ChangeLog:

	* gcc.dg/torture/pr127085.c: New test.