[RISCV] Mark Sub/AddChainWithSubs as legal reduction types (#154753)

We used to vectorize these scalably but after #147026 they were split
out from RecurKind::Add into their own RecurKinds, and we didn't mark
them as supported in isLegalToVectorizeReduction.

This caused the loop vectorizer to drop the scalable VPlan because it
thinks the reductions will be scalarized.

This fixes it by just marking them as supported.

Fixes #154554

Author: lukel97

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h

@@ -372,6 +372,8 @@ class RISCVTTIImpl final : public BasicTTIImplBase<RISCVTTIImpl> {
 
     switch (RdxDesc.getRecurrenceKind()) {
     case RecurKind::Add:
+    case RecurKind::Sub:
+    case RecurKind::AddChainWithSubs:
     case RecurKind::And:
     case RecurKind::Or:
     case RecurKind::Xor: