[SystemZ] Add use of super-reg in splitMove()

If one of the subregs of the 128 bit reg is undefined when splitMove() splits
a store into two instructions, a use of an undefined physical register
results.

To remedy this, an implicit use of the super register is added onto both new
instructions, along with propagated kill and undef flags.

This was discovered with llvm-stress, and that test case is attached as
test/CodeGen/SystemZ/splitMove_undefReg_mverifier.ll

Thanks to Matthias Braun for helping with a nice explanation.

Review: Ulrich Weigand
llvm-svn: 298047