[SystemZ] Fix assertion failure in adjustSubwordCmp

When comparing a zero-extended value against a constant small enough to
be in range of the inner type, it doesn't matter whether a signed or
unsigned compare operation (for the outer type) is being used.  This is
why the code in adjustSubwordCmp had this assertion:

    assert(C.ICmpType == SystemZICMP::Any &&
           "Signedness shouldn't matter here.");

assuming the the caller had already detected that fact.  However, it
turns out that there cases, in particular with always-true or always-
false conditions that have not been eliminated when compiling at -O0,
where this is not true.

Instead of failing an assertion if C.ICmpType is not SystemZICMP::Any
here, we can simply *set* it safely to SystemZICMP::Any, however.

llvm-svn: 255786