[VPlanSLP] Don't dereference a cast_or_null<VPInstruction> result. NFCI.

The static analyzer is warning about a potential null dereference of the cast_or_null result, I've split the cast_or_null check from the ->getUnderlyingInstr() call to avoid this, but it appears that we weren't seeing any null pointers in the dumped bundles in the first place.

llvm-svn: 371975
diff --git a/llvm/lib/Transforms/Vectorize/VPlanSLP.cpp b/llvm/lib/Transforms/Vectorize/VPlanSLP.cpp
index e5ab24e..9019ed1 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanSLP.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanSLP.cpp
@@ -346,11 +346,14 @@
 
 void VPlanSlp::dumpBundle(ArrayRef<VPValue *> Values) {
   dbgs() << " Ops: ";
-  for (auto Op : Values)
-    if (auto *Instr = cast_or_null<VPInstruction>(Op)->getUnderlyingInstr())
-      dbgs() << *Instr << " | ";
-    else
-      dbgs() << " nullptr | ";
+  for (auto Op : Values) {
+    if (auto *VPInstr = cast_or_null<VPInstruction>(Op))
+      if (auto *Instr = VPInstr->getUnderlyingInstr()) {
+        dbgs() << *Instr << " | ";
+        continue;
+      }
+    dbgs() << " nullptr | ";
+  }
   dbgs() << "\n";
 }