[SLP] Optimize getSpillCost(); NFCI

For a given set of live values, the spill cost will always be the
same for each call. Compute the cost once and multiply it by the
number of calls.

(I'm not sure this spill cost modeling makes sense if there are
multiple calls, as the spill cost will likely be shared across
calls in that case. But that's how it currently works.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365552 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -3355,6 +3355,7 @@ int BoUpSLP::getSpillCost() const {
     });
 
     // Now find the sequence of instructions between PrevInst and Inst.
+    unsigned NumCalls = 0;
     BasicBlock::reverse_iterator InstIt = ++Inst->getIterator().getReverse(),
                                  PrevInstIt =
                                      PrevInst->getIterator().getReverse();
@@ -3367,16 +3368,19 @@ int BoUpSLP::getSpillCost() const {
       // Debug informations don't impact spill cost.
       if ((isa<CallInst>(&*PrevInstIt) &&
            !isa<DbgInfoIntrinsic>(&*PrevInstIt)) &&
-          &*PrevInstIt != PrevInst) {
-        SmallVector<Type*, 4> V;
-        for (auto *II : LiveValues)
-          V.push_back(VectorType::get(II->getType(), BundleWidth));
-        Cost += TTI->getCostOfKeepingLiveOverCall(V);
-      }
+          &*PrevInstIt != PrevInst)
+        NumCalls++;
 
       ++PrevInstIt;
     }
 
+    if (NumCalls) {
+      SmallVector<Type*, 4> V;
+      for (auto *II : LiveValues)
+        V.push_back(VectorType::get(II->getType(), BundleWidth));
+      Cost += NumCalls * TTI->getCostOfKeepingLiveOverCall(V);
+    }
+
     PrevInst = Inst;
   }