Define s390_shift_truncation_mask to allow the optabs optimization

    sh = (64 - sh)
 -> sh = -sh

for a rotation operation.

gcc/ChangeLog:

2019-07-08  Robin Dapp  <rdapp@linux.ibm.com>

	* config/s390/s390.c (s390_shift_truncation_mask): Define.
	(TARGET_SHIFT_TRUNCATION_MASK): Define.

gcc/testsuite/ChangeLog:

2019-07-08  Robin Dapp  <rdapp@linux.ibm.com>

	* gcc.target/s390/rotate-truncation-mask.c: New test.

From-SVN: r273237