tree-optimization/116463 - complex lowering leaves around dead stmts

tree-optimization/116463 - complex lowering leaves around dead stmts

Complex lowering generally replaces existing complex defs with
COMPLEX_EXPRs but those might be dead when it can always refer to
components from the lattice.  This in turn can pessimize followup
transforms like forwprop and reassoc, the following makes sure to
get rid of dead COMPLEX_EXPRs generated by using
simple_dce_from_worklist.

	PR tree-optimization/116463
	* tree-complex.cc: Include tree-ssa-dce.h.
	(dce_worklist): New global.
	(update_complex_assignment): Add SSA def to the DCE worklist.
	(tree_lower_complex): Perform DCE.