Add 'final' and 'override' on dom_walker vfunc impls

Add 'final' and 'override' on dom_walker vfunc impls

gcc/ChangeLog:
	* compare-elim.cc: Add "final" and "override" to dom_walker vfunc
	implementations, removing redundant "virtual" as appropriate.
	* gimple-ssa-strength-reduction.cc: Likewise.
	* ipa-prop.cc: Likewise.
	* rtl-ssa/blocks.cc: Likewise.
	* tree-into-ssa.cc: Likewise.
	* tree-ssa-dom.cc: Likewise.
	* tree-ssa-math-opts.cc: Likewise.
	* tree-ssa-phiopt.cc: Likewise.
	* tree-ssa-propagate.cc: Likewise.
	* tree-ssa-sccvn.cc: Likewise.
	* tree-ssa-strlen.cc: Likewise.
	* tree-ssa-uncprop.cc: Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>