[mlir][vector] Disable from_elements for scalable vectors (#117868)

Disables `vector.from_elements` for scalable vectors. Given that the
length of scalable vectors is unknown at compile time, the semantics of
this Op are unclear in this context.

Author: banach-space

mlir/include/mlir/Dialect/Vector/IR/VectorOps.td

@@ -806,10 +806,12 @@ def Vector_FromElementsOp : Vector_Op<"from_elements", [
     // [[[%f1, %f2]], [[%f3, %f4]], [[%f5, %f6]]]
     %3 = vector.from_elements %f1, %f2, %f3, %f4, %f5, %f6 : vector<3x1x2xf32>
     ```
+
+    Note, scalable vectors are not supported.
   }];
 
   let arguments = (ins Variadic<AnyType>:$elements);
-  let results = (outs AnyVectorOfAnyRank:$result);
+  let results = (outs AnyFixedVectorOfAnyRank:$result);
   let assemblyFormat = "$elements attr-dict `:` type($result)";
   let hasCanonicalizer = 1;
 }

mlir/test/Dialect/Vector/invalid.mlir

@@ -1803,6 +1803,14 @@ func.func @invalid_from_elements(%a: f32, %b: i32) {
 
 // -----
 
+func.func @invalid_from_elements_scalable(%a: f32, %b: i32) {
+  // expected-error @+1 {{'result' must be fixed-length vector of any type values, but got 'vector<[2]xf32>'}}
+  vector.from_elements %a, %b : vector<[2]xf32>
+  return
+}
+
+// -----
+
 func.func @invalid_step_0d() {
   // expected-error @+1 {{vector.step' op result #0 must be vector of index values of ranks 1, but got 'vector<f32>'}}
   vector.step : vector<f32>