[SystemZ] Use "let Predicates =" for blocks of new instructions

...instead of a separate Requires for each one.  This style was already
used in some places and seems more compact.

No behavioral change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204452 91177308-0d34-0410-b5e6-96231b3b80d8