[SystemZ] Refactor some VT casts in DAG match patterns

In patterns where we need to specify a result VT, prefer

  [(set (tr.vt tr.op:$V1), (operator ...))]

over

  [(set tr.op:$V1, (tr.vt (operator ...)))]

This is NFC now, but simplifies some future changes.

llvm-svn: 331192