LRA doesn't like the 'X' constraint as used in our tests, remove it.

gcc/testsuite
2017-07-10  Claudiu Zissulescu  <claziss@synopsys.com>

	* gcc.target/arc/mulsi3_highpart-1.c: Remove 'X' constraint.
	* gcc.target/arc/mulsi3_highpart-2.c: Likewise.

From-SVN: r250096