[SLP] Optimize stacksave dependence handling [NFC]

After writing the commit message for 4b1bace28, realized that the mentioned optimization was rather straight forward.  We already have the code for scanning a block during region initialization, we can simply keep track if we've seen a stacksave or stackrestore.  If we haven't, none of these dependencies are relevant and we can avoid the relatively expensive scans entirely.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -2764,6 +2764,7 @@ private:
       ScheduleEnd = nullptr;
       FirstLoadStoreInRegion = nullptr;
       LastLoadStoreInRegion = nullptr;
+      RegionHasStackSave = false;
 
       // Reduce the maximum schedule region size by the size of the
       // previous scheduling run.
@@ -3032,6 +3033,11 @@ private:
     /// (can be null).
     ScheduleData *LastLoadStoreInRegion = nullptr;
 
+    /// Is there an llvm.stacksave or llvm.stackrestore in the scheduling
+    /// region?  Used to optimize the dependence calculation for the
+    /// common case where there isn't.
+    bool RegionHasStackSave = false;
+
     /// The current size of the scheduling region.
     int ScheduleRegionSize = 0;
 
@@ -8016,6 +8022,10 @@ void BoUpSLP::BlockScheduling::initScheduleData(Instruction *FromI,
       }
       CurrentLoadStore = SD;
     }
+
+    if (match(I, m_Intrinsic<Intrinsic::stacksave>()) ||
+        match(I, m_Intrinsic<Intrinsic::stackrestore>()))
+      RegionHasStackSave = true;
   }
   if (NextLoadStore) {
     if (CurrentLoadStore)
@@ -8099,6 +8109,7 @@ void BoUpSLP::BlockScheduling::calculateDependencies(ScheduleData *SD,
         }
       }
 
+      if (RegionHasStackSave) {
         // If we have an inalloc alloca instruction, it needs to be scheduled
         // after any preceeding stacksave.  We also need to prevent any alloca
         // from reordering above a preceeding stackrestore.
@@ -8135,6 +8146,7 @@ void BoUpSLP::BlockScheduling::calculateDependencies(ScheduleData *SD,
             break;
           }
         }
+      }
 
       // Handle the memory dependencies (if any).
       ScheduleData *DepDest = BundleMember->NextLoadStore;