[SystemZ] Add a check against zero before calling getTestUnderMaskCond()

Csmith discovered that this function can be called with a zero argument,
in which case an assert for this triggered.

This patch also adds a guard before the other call to this function since
it was missing, although the test only covers the case where it was
discovered.

Reduced test case attached as CodeGen/SystemZ/int-cmp-54.ll.

Review: Ulrich Weigand
llvm-svn: 306287