fold-const.cc: remove strict_overflow_p from tree_binary_nonzero_warnv_p

This helper is called by tree_expr_nonzero_warnv_p(), a helper that is
gating a call to fold_overflow_warning() that we want to eliminate.

The helper is renamed to remove the "warn" from its name since it
doesn't throw or set any overflow warnings anymore.

gcc/ChangeLog:

	* fold-const.cc (tree_expr_nonzero_warnv_p): Removed
	strict_overflow_p flag from tree_binary_nonzero_p call.
	(tree_binary_nonzero_warnv_p): Renamed to tree_binary_nonzero_p.
	(tree_binary_nonzero_p): Removed strict_overflow_p flag.
	* fold-const.h (tree_binary_nonzero_warnv_p): Renamed to
	tree_binary_nonzero_p.
	(tree_binary_nonzero_p): Removed strict_overflow_p flag.