rs6000: Use unspec_volatile for darn (PR91481)

rs6000: Use unspec_volatile for darn (PR91481)

Every call to darn should deliver a *new* random number; such calls
should not be CSEd together.  So they should be unspec_volatile, not
plain unspec.

	PR target/91481
	* config/rs6000/rs6000.md (unspec): Delete UNSPEC_DARN, UNSPEC_DARN_32,
	and UNSPEC_DARN_RAW.
	(unspecv): New enumerator values UNSPECV_DARN, UNSPECV_DARN_32, and
	UNSPECV_DARN_RAW.
	(darn_32): Use an unspec_volatile, and UNSPECV_DARN_32.
	(darn_raw): Use an unspec_volatile, and UNSPECV_DARN_RAW.
	(darn): Use an unspec_volatile, and UNSPECV_DARN.

From-SVN: r274835