tree-optimization/111294 - backwards threader PHI costing

This revives an earlier patch since the problematic code applying
extra costs to PHIs in copied blocks we couldn't make any sense of
prevents a required threading in this case.  Instead of coming up
with an artificial other costing the following simply removes the
bits.

As with all threading changes this requires a plethora of testsuite
adjustments, but only the last three are unfortunate as is the
libgomp team.c adjustment which is required to avoid a bogus -Werror
diagnostic during bootstrap.

	PR tree-optimization/111294
gcc/
	* tree-ssa-threadbackward.cc (back_threader_profitability::m_name):
	Remove
	(back_threader::find_paths_to_names): Adjust.
	(back_threader::maybe_thread_block): Likewise.
	(back_threader_profitability::possibly_profitable_path_p): Remove
	code applying extra costs to copies PHIs.

libgomp/
	* team.c (gomp_team_start): Assert alloca size to avoid false
	positive alloc-size diagnostic.

gcc/testsuite/
	* gcc.dg/tree-ssa/pr111294.c: New test.
	* gcc.dg/tree-ssa/phi_on_compare-4.c: Adjust.
	* gcc.dg/tree-ssa/pr59597.c: Likewise.
	* gcc.dg/tree-ssa/pr61839_2.c: Likewise.
	* gcc.dg/tree-ssa/ssa-sink-18.c: Likewise.
	* g++.dg/warn/Wstringop-overflow-4.C: XFAIL subtest on ilp32.
	* gcc.dg/uninit-pred-9_b.c: XFAIL subtest everywhere.
	* gcc.dg/vect/vect-117.c: Make scan for not Invalid sum
	conditional on lp64.