[mlir][vector] Restrict vector.shape_cast (scalable vectors) (#100331)

Updates the verifier for `vector.shape_cast` so that incorrect cases
where "scalability" is dropped are immediately rejected. For example:
```mlir
  vector.shape_cast %vec : vector<1x1x[4]xindex> to vector<4xindex>
```

Also, as a separate PR, I've prepared a fix for the Linalg vectorizer to
avoid generating such shape casts (*):
* #100325

(*) Note, that's just one specific case that I've identified so far.

mlir/include/mlir/IR/BuiltinTypes.td

@@ -1168,6 +1168,11 @@ def Builtin_Vector : Builtin_Type<"Vector", "vector",
       return !llvm::is_contained(getScalableDims(), false);
     }
 
+    /// Get the number of scalable dimensions.
+    int64_t getNumScalableDims() const {
+      return llvm::count(getScalableDims(), true);
+    }
+
     /// Get or create a new VectorType with the same shape as `this` and an
     /// element type of bitwidth scaled by `scale`.
     /// Return null if the scaled element type cannot be represented.

mlir/lib/Dialect/Vector/IR/VectorOps.cpp

@@ -5238,6 +5238,16 @@ static LogicalResult verifyVectorShapeCast(Operation *op,
     if (!isValidShapeCast(resultShape, sourceShape))
       return op->emitOpError("invalid shape cast");
   }
+
+  // Check that (non-)scalability is preserved
+  int64_t sourceNScalableDims = sourceVectorType.getNumScalableDims();
+  int64_t resultNScalableDims = resultVectorType.getNumScalableDims();
+  if (sourceNScalableDims != resultNScalableDims)
+    return op->emitOpError("different number of scalable dims at source (")
+           << sourceNScalableDims << ") and result (" << resultNScalableDims
+           << ")";
+  sourceVectorType.getNumDynamicDims();
+
   return success();
 }
 

mlir/test/Dialect/Vector/invalid.mlir

@@ -1182,6 +1182,20 @@ func.func @shape_cast_invalid_rank_expansion(%arg0 : vector<15x2xf32>) {
 
 // -----
 
+func.func @shape_cast_scalability_flag_is_dropped(%arg0 : vector<15x[2]xf32>) {
+  // expected-error@+1 {{different number of scalable dims at source (1) and result (0)}}
+  %0 = vector.shape_cast %arg0 : vector<15x[2]xf32> to vector<30xf32>
+}
+
+// -----
+
+func.func @shape_cast_scalability_flag_is_dropped(%arg0 : vector<2x[15]x[2]xf32>) {
+  // expected-error@+1 {{different number of scalable dims at source (2) and result (1)}}
+  %0 = vector.shape_cast %arg0 : vector<2x[15]x[2]xf32> to vector<30x[2]xf32>
+}
+
+// -----
+
 func.func @bitcast_not_vector(%arg0 : vector<5x1x3x2xf32>) {
   // expected-error@+1 {{'vector.bitcast' invalid kind of type specified}}
   %0 = vector.bitcast %arg0 : vector<5x1x3x2xf32> to f32