Simplify; NFC

Not shown in the diff: AQ is a `vector<SUnit *>`, and SU is a `SUnit *`

llvm-svn: 334451
diff --git a/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp b/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
index 1dfd840..dbe83bc 100644
--- a/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
+++ b/llvm/lib/Target/AMDGPU/R600MachineScheduler.cpp
@@ -459,7 +459,7 @@
   }
   if (!AQ.empty()) {
     SU = AQ.back();
-    AQ.resize(AQ.size() - 1);
+    AQ.pop_back();
   }
   return SU;
 }