PR optimization/8423
	* cse.c (fold_rtx): Only eliminate a CONSTANT_P_RTX to 1 when
	its argument is constant, or 0 if !flag_gcse.
	* simplify-rtx.c (simplify_rtx): Convert CONSTANT_P_RTX to 1
	if it's argument is constant.
	* gcse.c (want_to_gcse_p): Ignore CONSTANT_P_RTX nodes.
	(hash_scan_set): Don't record CONSTANT_P_RTX expressions.
	(do_local_cprop): Don't propagate CONSTANT_P_RTX constants.
	* builtins.c (purge_builtin_constant_p): New function to force
	instantiation of any remaining CONSTANT_P_RTX nodes.
	* rtl.h (purge_builtin_constant_p): Prototype here.
	* toplev.c (rest_of_compilation): Invoke purge_builtin_constant_p
	pass after GCSE and before loop.
	(flag_gcse): No longer static.
	* flags.h (flag_gcse): Prototype here.

From-SVN: r61642