PHIOPT: Improve replace_phi_edge_with_variable's dce_ssa_names slightly

When I added the dce_ssa_names argument, I didn't realize bitmap was a
pointer so I used the default argument value as auto_bitmap(). But
instead we could just use nullptr and check if it was a nullptr
before calling simple_dce_from_worklist.

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (replace_phi_edge_with_variable): Change
	the default argument value for dce_ssa_names to nullptr.
	Check to make sure dce_ssa_names is a non-nullptr before
	calling simple_dce_from_worklist.