IBM Z: Remove match_scratch workaround

IBM Z: Remove match_scratch workaround

Since commit dd1ef00c45ba ("Fix bug in the define_subst handling that
made match_scratch unusable for multi-alternative patterns.") the
workaround for that bug in *ashrdi3_31<setcc><cconly> is not only no
longer necessary, but actually breaks the build.

Get rid of it by using only one alternative in (match_scratch).  It
will be replicated as many times as needed in order to match the
pattern with which (define_subst) is used.

gcc/ChangeLog:

	* config/s390/s390.md(*ashrdi3_31<setcc><cconly>): Use a single
	constraint.
	* config/s390/subst.md(cconly_subst): Use a single constraint
	in (match_scratch).

gcc/testsuite/ChangeLog:

	* gcc.target/s390/ashr.c: New test.