[SystemZ] Use subregs for 64-bit truncating stores

Another patch to reduce the duplication of encoding information.
Rather than define separate patterns for truncating 64-bit stores,
use the 32-bit stores with a subreg.  No behavioral changed intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191365 91177308-0d34-0410-b5e6-96231b3b80d8