Relax the only one back-edge restriction.

The rule is in the way for better register allocation, as
it creates an artificial join point between multiple paths.

Change-Id: Ia4392890f95bcea56d143138f28ddce6c572ad58
diff --git a/compiler/optimizing/liveness_test.cc b/compiler/optimizing/liveness_test.cc
index 8a96ee9..1914339 100644
--- a/compiler/optimizing/liveness_test.cc
+++ b/compiler/optimizing/liveness_test.cc
@@ -445,44 +445,40 @@
 
 TEST(LivenessTest, Loop6) {
   // Bitsets are made of:
-  // (constant0, constant4, constant5, phi in block 2, phi in block 8)
+  // (constant0, constant4, constant5, phi in block 2)
   const char* expected =
     "Block 0\n"
-    "  live in: (00000)\n"
-    "  live out: (11100)\n"
-    "  kill: (11100)\n"
+    "  live in: (0000)\n"
+    "  live out: (1110)\n"
+    "  kill: (1110)\n"
     "Block 1\n"
-    "  live in: (11100)\n"
-    "  live out: (01100)\n"
-    "  kill: (00000)\n"
+    "  live in: (1110)\n"
+    "  live out: (0110)\n"
+    "  kill: (0000)\n"
     "Block 2\n"  // loop header
-    "  live in: (01100)\n"
-    "  live out: (01110)\n"
-    "  kill: (00010)\n"
+    "  live in: (0110)\n"
+    "  live out: (0111)\n"
+    "  kill: (0001)\n"
     "Block 3\n"
-    "  live in: (01100)\n"
-    "  live out: (01100)\n"
-    "  kill: (00000)\n"
-    "Block 4\n"  // original back edge
-    "  live in: (01100)\n"
-    "  live out: (01100)\n"
-    "  kill: (00000)\n"
-    "Block 5\n"  // original back edge
-    "  live in: (01100)\n"
-    "  live out: (01100)\n"
-    "  kill: (00000)\n"
+    "  live in: (0110)\n"
+    "  live out: (0110)\n"
+    "  kill: (0000)\n"
+    "Block 4\n"  // back edge
+    "  live in: (0110)\n"
+    "  live out: (0110)\n"
+    "  kill: (0000)\n"
+    "Block 5\n"  // back edge
+    "  live in: (0110)\n"
+    "  live out: (0110)\n"
+    "  kill: (0000)\n"
     "Block 6\n"  // return block
-    "  live in: (00010)\n"
-    "  live out: (00000)\n"
-    "  kill: (00000)\n"
+    "  live in: (0001)\n"
+    "  live out: (0000)\n"
+    "  kill: (0000)\n"
     "Block 7\n"  // exit block
-    "  live in: (00000)\n"
-    "  live out: (00000)\n"
-    "  kill: (00000)\n"
-    "Block 8\n"  // synthesized back edge
-    "  live in: (01100)\n"
-    "  live out: (01100)\n"
-    "  kill: (00001)\n";
+    "  live in: (0000)\n"
+    "  live out: (0000)\n"
+    "  kill: (0000)\n";
 
   const uint16_t data[] = ONE_REGISTER_CODE_ITEM(
     Instruction::CONST_4 | 0 | 0,