[VPlan] Clarify legality check in licm (NFC) (#162486)

Recipes in licm are safe to hoist if the legality check passes, and the
recipe is guaranteed to execute; the single successor of the vector
preheader is the vector loop region. Clarify this in the code structure
and comments.

Author: artagnon

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -2122,27 +2122,32 @@ static void licm(VPlan &Plan) {
   VPBasicBlock *Preheader = Plan.getVectorPreheader();
 
   // Return true if we do not know how to (mechanically) hoist a given recipe
-  // out of a loop region. Does not address legality concerns such as aliasing
-  // or speculation safety.
+  // out of a loop region.
   auto CannotHoistRecipe = [](VPRecipeBase &R) {
+    // TODO: Relax checks in the future, e.g. we could also hoist reads, if
+    // their memory location is not modified in the vector loop.
+    if (R.mayHaveSideEffects() || R.mayReadFromMemory() || R.isPhi())
+      return true;
+
     // Allocas cannot be hoisted.
     auto *RepR = dyn_cast<VPReplicateRecipe>(&R);
     return RepR && RepR->getOpcode() == Instruction::Alloca;
   };
 
   // Hoist any loop invariant recipes from the vector loop region to the
   // preheader. Preform a shallow traversal of the vector loop region, to
-  // exclude recipes in replicate regions.
+  // exclude recipes in replicate regions. Since the top-level blocks in the
+  // vector loop region are guaranteed to execute if the vector pre-header is,
+  // we don't need to check speculation safety.
   VPRegionBlock *LoopRegion = Plan.getVectorLoopRegion();
+  assert(Preheader->getSingleSuccessor() == LoopRegion &&
+         "Expected vector prehader's successor to be the vector loop region");
   for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
            vp_depth_first_shallow(LoopRegion->getEntry()))) {
     for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
       if (CannotHoistRecipe(R))
         continue;
-      // TODO: Relax checks in the future, e.g. we could also hoist reads, if
-      // their memory location is not modified in the vector loop.
-      if (R.mayHaveSideEffects() || R.mayReadFromMemory() || R.isPhi() ||
-          any_of(R.operands(), [](VPValue *Op) {
+      if (any_of(R.operands(), [](VPValue *Op) {
             return !Op->isDefinedOutsideLoopRegions();
           }))
         continue;