Fix test case.

r230921 broke backend-optimization-failure.cpp: after
r230921, LLVM no longer emits an expression to compute 'Length - 1'
and this perturbs LoopSimplify enough to emit the warning on line 10
instead of line 9.  This is a review request to fix the test case once
I re-land r230921.

llvm-svn: 231020