[RISCV] Adjust unroll prefs for loops with vectors (#151525)

Adjust the unrolling preferences to unroll hand-vectorized code, as well
as the scalar remainder of a vectorized loop. Inspired by a similar
effort in AArch64: see #147420 and #151164.

Author: artagnon

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2627,18 +2627,17 @@ void RISCVTTIImpl::getUnrollingPreferences(
   if (L->getNumBlocks() > 4)
     return;
 
-  // Don't unroll vectorized loops, including the remainder loop
-  if (getBooleanLoopAttribute(L, "llvm.loop.isvectorized"))
-    return;
-
   // Scan the loop: don't unroll loops with calls as this could prevent
-  // inlining.
+  // inlining. Don't unroll auto-vectorized loops either, though do allow
+  // unrolling of the scalar remainder.
+  bool IsVectorized = getBooleanLoopAttribute(L, "llvm.loop.isvectorized");
   InstructionCost Cost = 0;
   for (auto *BB : L->getBlocks()) {
     for (auto &I : *BB) {
-      // Initial setting - Don't unroll loops containing vectorized
-      // instructions.
-      if (I.getType()->isVectorTy())
+      // Both auto-vectorized loops and the scalar remainder have the
+      // isvectorized attribute, so differentiate between them by the presence
+      // of vector instructions.
+      if (IsVectorized && I.getType()->isVectorTy())
         return;
 
       if (isa<CallInst>(I) || isa<InvokeInst>(I)) {