[VectorUtils] Improve computeMinimumValueSizes (NFC)

The Worklist used by computeMinimumValueSizes is wastefully a
Value-vector, when the top of the loop attempts to cast the popped value
to an Instruction anyway. Improve the algorithm by cutting this wasteful
work, refining the type of Worklist, Visited, and Roots from
Value-vectors to Instruction-vectors.

Author: artagnon

llvm/lib/Analysis/VectorUtils.cpp

@@ -750,9 +750,9 @@ llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
   // to ensure no extra casts would need to be inserted, so every DAG
   // of connected values must have the same minimum bitwidth.
   EquivalenceClasses<Value *> ECs;
-  SmallVector<Value *, 16> Worklist;
-  SmallPtrSet<Value *, 4> Roots;
-  SmallPtrSet<Value *, 16> Visited;
+  SmallVector<Instruction *, 16> Worklist;
+  SmallPtrSet<Instruction *, 4> Roots;
+  SmallPtrSet<Instruction *, 16> Visited;
   DenseMap<Value *, uint64_t> DBits;
   SmallPtrSet<Instruction *, 4> InstructionSet;
   MapVector<Instruction *, uint64_t> MinBWs;
@@ -786,17 +786,12 @@ llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
 
   // Now proceed breadth-first, unioning values together.
   while (!Worklist.empty()) {
-    Value *Val = Worklist.pop_back_val();
-    Value *Leader = ECs.getOrInsertLeaderValue(Val);
+    Instruction *I = Worklist.pop_back_val();
+    Value *Leader = ECs.getOrInsertLeaderValue(I);
 
-    if (!Visited.insert(Val).second)
+    if (!Visited.insert(I).second)
       continue;
 
-    // Non-instructions terminate a chain successfully.
-    if (!isa<Instruction>(Val))
-      continue;
-    Instruction *I = cast<Instruction>(Val);
-
     // If we encounter a type that is larger than 64 bits, we can't represent
     // it so bail out.
     if (DB.getDemandedBits(I).getBitWidth() > 64)
@@ -831,9 +826,10 @@ llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
       // All bits demanded, no point continuing.
       continue;
 
-    for (Value *O : cast<User>(I)->operands()) {
+    for (Value *O : I->operands()) {
       ECs.unionSets(Leader, O);
-      Worklist.push_back(O);
+      if (auto *OI = dyn_cast<Instruction>(O))
+        Worklist.push_back(OI);
     }
   }
 
@@ -874,7 +870,7 @@ llvm::computeMinimumValueSizes(ArrayRef<BasicBlock *> Blocks, DemandedBits &DB,
       if (!MI)
         continue;
       Type *Ty = M->getType();
-      if (Roots.count(M))
+      if (Roots.count(MI))
         Ty = MI->getOperand(0)->getType();
 
       if (MinBW >= Ty->getScalarSizeInBits())