[SSAUpdater] Avoid scanning basic blocks to find instruction order.

This fixes a compile-time regression caused by llvm#116645, where an entry
basic block with a very large number of allocas and other instructions
caused SROA to take ~100× its expected runtime, as every alloca (with ~2
uses) now calls this method to find the order of those few instructions,
rescanning the very large basic block every single time.

Since this code was originally written, Instructions now have ordering
numbers available to determine relative order without unnecessarily
scanning the basic block.

Author: jcranmer-intel

llvm/lib/Transforms/Utils/SSAUpdater.cpp

@@ -432,26 +432,30 @@ void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
       }
     }
 
-    // If so, we can queue them all as live in loads.  We don't have an
-    // efficient way to tell which on is first in the block and don't want to
-    // scan large blocks, so just add all loads as live ins.
+    // If so, we can queue them all as live in loads.
     if (!HasStore) {
       for (Instruction *I : BlockUses)
         LiveInLoads.push_back(cast<LoadInst>(I));
       BlockUses.clear();
       continue;
     }
 
+    // Sort all of the interesting instructions in the block so that we don't
+    // have to scan a large block just to find a few instructions.
+    std::sort(BlockUses.begin(), BlockUses.end(),
+              [](Instruction *A, Instruction *B) { return A->comesBefore(B); });
+
     // Otherwise, we have mixed loads and stores (or just a bunch of stores).
     // Since SSAUpdater is purely for cross-block values, we need to determine
     // the order of these instructions in the block.  If the first use in the
     // block is a load, then it uses the live in value.  The last store defines
-    // the live out value.  We handle this by doing a linear scan of the block.
+    // the live out value.
     Value *StoredValue = nullptr;
-    for (Instruction &I : *BB) {
-      if (LoadInst *L = dyn_cast<LoadInst>(&I)) {
+    for (Instruction *I : BlockUses) {
+      if (LoadInst *L = dyn_cast<LoadInst>(I)) {
         // If this is a load from an unrelated pointer, ignore it.
-        if (!isInstInList(L, Insts)) continue;
+        if (!isInstInList(L, Insts))
+          continue;
 
         // If we haven't seen a store yet, this is a live in use, otherwise
         // use the stored value.
@@ -465,14 +469,15 @@ void LoadAndStorePromoter::run(const SmallVectorImpl<Instruction *> &Insts) {
         continue;
       }
 
-      if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
+      if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
         // If this is a store to an unrelated pointer, ignore it.
-        if (!isInstInList(SI, Insts)) continue;
+        if (!isInstInList(SI, Insts))
+          continue;
         updateDebugInfo(SI);
 
         // Remember that this is the active value in the block.
         StoredValue = SI->getOperand(0);
-      } else if (auto *AI = dyn_cast<AllocaInst>(&I)) {
+      } else if (auto *AI = dyn_cast<AllocaInst>(I)) {
         // Check if this an alloca, in which case we treat it as a store of
         // getValueToUseForAlloca.
         if (!isInstInList(AI, Insts))