Adjust the backend-optimization-failure.cpp test case

[sanjoy]

Summary

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.

[majnemer]

LGTM

[sanjoy]

Was checked in in rL231020.

test/Misc/backend-optimization-failure.cpp

 // RUN: %clang_cc1 -triple x86_64-unknown-unknown %s -O3 -emit-llvm -gline-tables-only -S -verify -o /dev/null
 // REQUIRES: x86-registered-target
 
 // Test verifies optimization failures generated by the backend are handled
 // correctly by clang. LLVM tests verify all of the failure conditions.
 
 void test_switch(int *A, int *B, int Length) {
 #pragma clang loop vectorize(enable) unroll(disable)
-  /* expected-warning {{loop not vectorized: failed explicitly specified loop vectorization}} */ for (int i = 0; i < Length; i++) {
-    switch (A[i]) {
+  for (int i = 0; i < Length; i++) {
+/* expected-warning {{loop not vectorized: failed explicitly specified loop vectorization}} */ switch (A[i]) {
     case 0:
       B[i] = 1;
       break;
     case 1:
       B[i] = 2;
       break;
     default:
       B[i] = 3;
     }
   }
 }