Implement POINTER_DIFF_EXPR entry in range-op.

Implement POINTER_DIFF_EXPR entry in range-op.

I've seen cases in the upcoming jump threader enhancements where we see
a difference of two pointers that are known to be equivalent, and yet we
fail to return 0 for the range.  This is because we have no working
range-op entry for POINTER_DIFF_EXPR.  The entry we currently have is
a mere placeholder to avoid ignoring POINTER_DIFF_EXPR's so
adjust_pointer_diff_expr() could get a whack at it here:

//	def = __builtin_memchr (arg, 0, sz)
//	n = def - arg
//
// The range for N can be narrowed to [0, PTRDIFF_MAX - 1].

This patch adds the relational magic to range-op, which we can just
steal from the minus_expr code.

gcc/ChangeLog:

	* range-op.cc (operator_minus::op1_op2_relation_effect): Abstract
	out to...
	(minus_op1_op2_relation_effect): ...here.
	(class operator_pointer_diff): New.
	(operator_pointer_diff::op1_op2_relation_effect): Call
	minus_op1_op2_relation_effect.
	(integral_table::integral_table): Add entry for POINTER_DIFF_EXPR.