[SystemZ] Add a structure to represent a selected comparison

...in an attempt to rein back the increasingly complex selection code.
A knock-on effect is that ICmpType is exposed from the outset, which
slightly simplifies adjustSubwordCmp.

The code is no piece of art even after this change, but at least it should
be slightly better.  No behavioral change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197235 91177308-0d34-0410-b5e6-96231b3b80d8