[SystemZ] Fix LLVM crash on unoptimized code

[SystemZ] Fix LLVM crash on unoptimized code

Compiling the following function with -O0 would crash, since LLVM would
hit an assertion in getTestUnderMaskCond:

  int test(unsigned long x)
  {
    return x >= 0 && x <= 15;
  }

Fixed by detecting the case in the caller of getTestUnderMaskCond.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233541 91177308-0d34-0410-b5e6-96231b3b80d8