autopar TLC

The following removes all excessive update_ssa calls from OMP
expansion, thereby rewriting the atomic load and store cases to
GIMPLE code generation.  I don't think autopar ever exercises the
atomics code though.

There's not much test coverage overall so I've built SPEC 2k17
with -floop-parallelize-all -ftree-parallelize-loops=2 with and
without LTO (and otherwise -Ofast plus -march=haswell) without
fallout.

If there's any fallout it's not OK to update SSA form for
each and every OMP stmt lowered.

	* omp-expand.cc (expand_omp_atomic_load): Emit GIMPLE
	directly.  Avoid update_ssa when in SSA form.
	(expand_omp_atomic_store): Likewise.
	(expand_omp_atomic_fetch_op): Avoid update_ssa when in SSA
	form.
	(expand_omp_atomic_pipeline): Likewise.
	(expand_omp_atomic_mutex): Likewise.
	* tree-parloops.cc (gen_parallel_loop): Use
	TODO_update_ssa_no_phi after loop_version.