[TTI] Handle experimental.vp.reverse in BasicTTIImpl (#147868)

A experimental_vp_reverse isn't exactly functionally the same as
vector_reverse, so previously it wasn't getting picked up by the generic
VP costing code that reuses the non-VP equivalents.

But for costing purposes it's good enough so we can reuse it.

The type-based cost for vector_reverse is still incorrect and should be
fixed in another patch.

Author: lukel97

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -1787,6 +1787,11 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
 
       std::optional<Intrinsic::ID> FID =
           VPIntrinsic::getFunctionalIntrinsicIDForVP(ICA.getID());
+
+      // Not functionally equivalent but close enough for cost modelling.
+      if (ICA.getID() == Intrinsic::experimental_vp_reverse)
+        FID = Intrinsic::vector_reverse;
+
       if (FID) {
         // Non-vp version will have same arg types except mask and vector
         // length.