tree-optimization/106498 - reduce SSA updates in autopar

The following reduces the number of SSA updates done during autopar
OMP expansion, specifically avoiding the cases that just add virtual
operands (where maybe none have been before) in dead regions of the CFG.

Instead virtual SSA update is delayed until after the pass.  There's
much more TLC needed here, but test coverage makes it really difficult.

	PR tree-optimization/106498
	* omp-expand.cc (expand_omp_taskreg): Do not perform virtual
	SSA update here.
	(expand_omp_for): Or here.
	(execute_expand_omp): Instead schedule it here together
	with CFG cleanup via TODO.