[SystemZ] Minor bugfixes.

LLCH, LLHH and CLIH had the wrong register classes for the def-operand.
Tie operands if changing opcode to an instruction with tied ops.
Comment typo fix.

These fixes were needed in order to make regression test case
SystemZ/asm-18.ll pass with -verify-machineinstrs (not used by
default).

Reviewed by Ulrich Weigand.

llvm-svn: 249811