[SystemZ] Extend combineGET_CCMASK() to handle a truncated SELECT_CCMASK.

In cases where the SELECT_CCMASK has an additional user of the carry, a
truncated SELECT_CCMASK may result as the input to the GET_CCMASK, which need
to be recognized.

Fixes https://github.com/llvm/llvm-project/issues/59054

Reviewed By: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D138324