[LV] Directly check if memory or SCEV check blocks are used (NFCI).

Slightly simplify the logic to retrieve check blocks in
GeneratedRTChecks, to prepare for additional refactoring.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1768,15 +1768,14 @@ class GeneratedRTChecks {
   BasicBlock *SCEVCheckBlock = nullptr;
 
   /// The value representing the result of the generated SCEV checks. If it is
-  /// nullptr, either no SCEV checks have been generated or they have been used.
+  /// nullptr no SCEV checks have been generated.
   Value *SCEVCheckCond = nullptr;
 
   /// Basic block which contains the generated memory runtime checks, if any.
   BasicBlock *MemCheckBlock = nullptr;
 
   /// The value representing the result of the generated memory runtime checks.
-  /// If it is nullptr, either no memory runtime checks have been generated or
-  /// they have been used.
+  /// If it is nullptr no memory runtime checks have been generated.
   Value *MemRuntimeCheckCond = nullptr;
 
   /// True if any checks have been added.
@@ -1996,13 +1995,14 @@ class GeneratedRTChecks {
   ~GeneratedRTChecks() {
     SCEVExpanderCleaner SCEVCleaner(SCEVExp);
     SCEVExpanderCleaner MemCheckCleaner(MemCheckExp);
-    if (!SCEVCheckCond)
+    bool SCEVChecksUsed = !SCEVCheckBlock || !pred_empty(SCEVCheckBlock);
+    bool MemChecksUsed = !MemCheckBlock || !pred_empty(MemCheckBlock);
+    if (SCEVChecksUsed)
       SCEVCleaner.markResultUsed();
 
-    if (!MemRuntimeCheckCond)
+    if (MemChecksUsed) {
       MemCheckCleaner.markResultUsed();
-
-    if (MemRuntimeCheckCond) {
+    } else {
       auto &SE = *MemCheckExp.getSE();
       // Memory runtime check generation creates compares that use expanded
       // values. Remove them before running the SCEVExpanderCleaners.
@@ -2016,9 +2016,9 @@ class GeneratedRTChecks {
     MemCheckCleaner.cleanup();
     SCEVCleaner.cleanup();
 
-    if (SCEVCheckCond)
+    if (!SCEVChecksUsed)
       SCEVCheckBlock->eraseFromParent();
-    if (MemRuntimeCheckCond)
+    if (!MemChecksUsed)
       MemCheckBlock->eraseFromParent();
   }
 
@@ -2044,8 +2044,6 @@ class GeneratedRTChecks {
     if (AddBranchWeights)
       setBranchWeights(BI, SCEVCheckBypassWeights, /*IsExpected=*/false);
     ReplaceInstWithInst(SCEVCheckBlock->getTerminator(), &BI);
-    // Mark the check as used, to prevent it from being removed during cleanup.
-    SCEVCheckCond = nullptr;
     AddedAnyChecks = true;
     return SCEVCheckBlock;
   }
@@ -2074,8 +2072,6 @@ class GeneratedRTChecks {
     MemCheckBlock->getTerminator()->setDebugLoc(
         Pred->getTerminator()->getDebugLoc());
 
-    // Mark the check as used, to prevent it from being removed during cleanup.
-    MemRuntimeCheckCond = nullptr;
     AddedAnyChecks = true;
     return MemCheckBlock;
   }