* tree-vrp.c (set_value_range_to_nonnegative): New function.
	(vrp_expr_computes_nonnegative, ssa_name_nonnegative_p): Likewise.
	(ssa_name_nonzero_p): Likewise.
	(get_value_range): Return NULL if VRP is not running.
	(extract_range_from_expr): Fallback to tree_expr_XXX_p if
	VRP routines do not discover a range.
	(vrp_finalize): Clear VR_VALUE to indicate VRP is not running.
	* tree.h (ssa_name_nonzero_p, ssa_name_nonnegative_p): Prototype.
	* fold-const.c (tree_expr_nonzero_p): For SSA_NAMEs, query VRP.
	(tree_expr_nonnegative_p): Similarly.
	* tree-ssa-dom.c (nonzero_vars, nonzero_vars_stack): Remove.
	(restore_nonzero_vars_to_original_value): Remove.
	(unsafe_associative_fp_binop): Remove.
	(tree_ssa_dominator_optimize): Remove initialization and
	finalization of nonzero_vars and nonzero_vars_stack.
	(dom_opt_initialize_block): No longer push marker on
	nonzero_vars_stack.
	(dom_opt_finalize_block): No longer call
	restore_nonzero_vars_to_original_value.
	(record_equivalences_from_phis): No longer look for
	nonzero PHI arguments.
	(cprop_into_successor_phis): No longer propagate nonzero
	property into PHI arguments.  Lose unused argument.  Caller
	updated.
	(record_equivalences_from_stmt): No longer record nonzero
	values for SSA_NAMEs.
	(lookup_avail_expr): No longer use nonzero_vars.

	* gcc.dg/tree-ssa/vrp24.c: Update expected output.
	* gcc.dg/tree-ssa/vrp26.c: New test.

From-SVN: r111175