Restore CCP copy propagation

The following restores copy propagation in CCP for the case the
lattice was constant before trying to transition to a copy.  At
some point we changed to use the meet operator to handle
integer constant -> integer constant transitions but that screws
up the const -> copy lattice transition.

	PR tree-optimization/84646
	* tree-ssa-ccp.cc (set_lattice_value): Make sure we
	allow a const -> copy transition and avoid using meet
	in that case.

	* gcc.dg/tree-ssa/ssa-ccp-42.c: New testcase.