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

Updates the verifier for `vector.shape_cast` so that the following
incorrect cases are immediately rejected:
```mlir
  vector.shape_cast %vec : vector<1x1x[4]xindex> to vector<4xindex>
```

Seperately, here's a fix for the Linalg vectorizer to prevent the
vectorizer from generating such shape casts (*):
* llvm#100325

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

Author: banach-space

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

@@ -5238,6 +5238,11 @@ static LogicalResult verifyVectorShapeCast(Operation *op,
     if (!isValidShapeCast(resultShape, sourceShape))
       return op->emitOpError("invalid shape cast");
   }
+
+  // Check that (non-)scalability is preserved
+  if (sourceVectorType.isScalable() != resultVectorType.isScalable())
+    return op->emitOpError("non-matching scalability flags");
+
   return success();
 }
 

mlir/test/Dialect/Vector/invalid.mlir

@@ -1182,6 +1182,13 @@ 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 {{non-matching scalability flags}}
+  %0 = vector.shape_cast %arg0 : vector<15x[2]xf32> to vector<30xf32>
+}
+
+// -----
+
 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