[llvm] [LV, VP]VP intrinsics support for the Loop Vectorizer + adding new tail-folding mode using EVL. (PR #76172)

[via llvm-commits]

================
@@ -350,6 +350,33 @@ Value *VPInstruction::generateInstruction(VPTransformState &State,
     Value *Zero = ConstantInt::get(ScalarTC->getType(), 0);
     return Builder.CreateSelect(Cmp, Sub, Zero);
   }
+  case VPInstruction::ExplicitVectorLength: {
+    // Compute EVL
+    auto GetEVL = [=](VPTransformState &State, Value *AVL) {
+      assert(AVL->getType()->isIntegerTy() &&
+             "Requested vector length should be an integer.");
+
+      // TODO: Add support for MaxSafeDist for correct loop emission.
+      assert(State.VF.isScalable() && "Expected scalable vector factor.");
+      Value *VFArg = State.Builder.getInt32(State.VF.getKnownMinValue());
+
+      Value *EVL = State.Builder.CreateIntrinsic(
+          State.Builder.getInt32Ty(), Intrinsic::experimental_get_vector_length,
+          {AVL, VFArg, State.Builder.getTrue()});
+      return EVL;
+    };
+    // TODO: Restructure this code with an explicit remainder loop, vsetvli can
+    // be outside of the main loop.
+    assert(Part == 0 && "No unrolling expected for predicated vectorization.");
+    // Compute VTC - IV as the EVL(requested vector length).
----------------
ayalz wrote:

```suggestion
    // Compute VTC - IV as the AVL (requested vector length).
```

https://github.com/llvm/llvm-project/pull/76172