PR rtl-optimization/18928
	* simplify_rtx.c (plus_minus_operand_p): New function to encode
	the test for suitable operands for calls to simplify_plus_minus.
	Only allow (CONST (PLUS x y)) if both x and y are CONSTANT_P.
	(simplify_binary_operation): Use plus_minus_operand_p.

	* gcc.dg/pr18928-1.c: New test case.

From-SVN: r92109