PR tree-optimization/29801
	* tree-ssa-ccp.c (get_symbol_constant_value): New function.
	(get_default_value): Use get_symbol_constant_value.
	(set_lattice_value): ICE when the value of the constant is
	changed.
	(visit_assignment): Ignore VDEFs of read-only variables.

	* gcc.dg/pr29801.c: New test.

From-SVN: r118926