match.pd ((convert (op:s (convert@2 @0) (convert?@3 @1)))): Add support for constant operand for OP.

	* match.pd ((convert (op:s (convert@2 @0) (convert?@3 @1)))): Add
	support for constant operand for OP.

	gcc/testsuite
	* gcc.dg/fold-narrowbopcst-1.c: New test.

From-SVN: r241624