[SystemZ] Add the CoveredBySubRegs bit to GPR64, GPR128 and FPR128 registers.

This bit is needed in order for the CalleeSavedRegs list to automatically
include the super registers if all of their subregs are present.

Thanks to Wei Mi for initially indicating this deficiency in the SystemZ
backend.

Review: Ulrich Weigand.
https://bugs.llvm.org/show_bug.cgi?id=34550

llvm-svn: 313023