[llvm] [LV][EVL]Support reversed loads/stores. (PR #88025)

[via llvm-commits]

================
@@ -9385,6 +9376,16 @@ void VPWidenLoadEVLRecipe::execute(VPTransformState &State) {
   Value *Mask = getMask()
                     ? State.get(getMask(), 0)
                     : Builder.CreateVectorSplat(State.VF, Builder.getTrue());
+  if (isReverse() && getMask()) {
+    VectorType *MaskTy = cast<VectorType>(Mask->getType());
+    Mask = Builder.CreateIntrinsic(
+        MaskTy, Intrinsic::experimental_vp_reverse,
+        {Mask,
+         Builder.CreateVectorSplat(MaskTy->getElementCount(),
+                                   Builder.getTrue()),
+         EVL},
+        nullptr, "vp.reverse.mask");
+  }
----------------
ayalz wrote:

The mask is arguably never masked itself, so reversing it should use an all-true mask.

Could a single AllTrueMask Value be reused, as in:
```suggestion
  Value *AllTrueMask = Builder.CreateVectorSplat(State.VF, Builder.getTrue());
  Value *Mask = getMask() ? State.get(getMask(), 0) : AllTrueMask; 
  if (isReverse() && getMask()) {
    VectorType *MaskTy = cast<VectorType>(Mask->getType());
    Mask = Builder.CreateIntrinsic(MaskTy, Intrinsic::experimental_vp_reverse,
                                  {Mask, AllTrueMask, EVL}, nullptr, "vp.reverse.mask");
  }
```

https://github.com/llvm/llvm-project/pull/88025