Add stats to simple_dce_from_worklist

While looking to move substitute_and_fold_engine
over to use simple_dce_from_worklist, I noticed
that we don't record the stats of the removed stmts/phis.
So this does that.

OK? Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-dce.cc (simple_dce_from_worklist): Record
	stats on removed number of statements and phis.