[SLP] NFC: Simplify CandidateVFs initialization (#144882)

Also adds a comment to clarify the meaning of MaxRegVF.

Author: sdesmalen-arm

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -21198,7 +21198,11 @@ bool SLPVectorizerPass::vectorizeStores(
         }
       }
 
+      // MaxRegVF represents the number of instructions (scalar, or vector in
+      // case of revec) that can be vectorized to naturally fit in a vector
+      // register.
       unsigned MaxRegVF = MaxVF;
+
       MaxVF = std::min<unsigned>(MaxVF, bit_floor(Operands.size()));
       if (MaxVF < MinVF) {
         LLVM_DEBUG(dbgs() << "SLP: Vectorization infeasible as MaxVF (" << MaxVF
@@ -21207,13 +21211,11 @@ bool SLPVectorizerPass::vectorizeStores(
         continue;
       }
 
-      unsigned Sz = 1 + Log2_32(MaxVF) - Log2_32(MinVF);
-      SmallVector<unsigned> CandidateVFs(Sz + (NonPowerOf2VF > 0 ? 1 : 0));
-      unsigned Size = MinVF;
-      for (unsigned &VF : reverse(CandidateVFs)) {
-        VF = Size > MaxVF ? NonPowerOf2VF : Size;
-        Size *= 2;
-      }
+      SmallVector<unsigned> CandidateVFs;
+      for (unsigned VF = std::max(MaxVF, NonPowerOf2VF); VF >= MinVF;
+           VF = divideCeil(VF, 2))
+        CandidateVFs.push_back(VF);
+
       unsigned End = Operands.size();
       unsigned Repeat = 0;
       constexpr unsigned MaxAttempts = 4;