[mlir][Conversion] Allow lowering to fixed arrays of scalable vectors

[benmxwl-arm]

Summary

This allows lowering vector types like: vector<3x[4]> or vector<3x2x[4]>
to LLVM IR, i.e. vectors where the trailing dim is scalable.

This is contingent on:
https://discourse.llvm.org/t/rfc-enable-arrays-of-scalable-vector-types/72935

More tests will be added in later patches, however, some MLIR fixes are
needed first.

Depends on: D158517

[benmxwl-arm]

• Remove dependency on (abandoned) VectorType helper methods

[awarzynski]

D158517 has just landed, so LGTM, thank you! 🙏🏻

[benmxwl-arm]

• Return failure rather than assert.

mlir/lib/Conversion/LLVMCommon/TypeConverter.cpp

Type LLVMTypeConverter::convertVectorType(VectorType type) const {
   if (!elementType)
     return {};
   if (type.getShape().empty())
     return VectorType::get({1}, elementType);
   Type vectorType = VectorType::get(type.getShape().back(), elementType,
                                     type.getScalableDims().back());
   assert(LLVM::isCompatibleVectorType(vectorType) &&
          "expected vector type compatible with the LLVM dialect");
-  assert(
-      (!type.isScalable() || (type.getRank() == 1)) &&
-      "expected 1-D scalable vector (n-D scalable vectors are not supported)");
+  assert(!type.dropBackDim().isScalable() &&
+         "only trailing dimension can be scalable");
   auto shape = type.getShape();
   for (int i = shape.size() - 2; i >= 0; --i)
     vectorType = LLVM::LLVMArrayType::get(vectorType, shape[i]);
   return vectorType;
 }
 
 /// Convert a type in the context of the default or bare pointer calling
 /// convention. Calling convention sensitive types, such as MemRefType and

mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir

 
 // CHECK-LABEL: @vector_scalable_extract
 // CHECK-SAME: %[[VEC:.*]]: vector<[4]xf32>
 func.func @vector_scalable_extract(%vec: vector<[4]xf32>) -> vector<8xf32> {
   // CHECK-NEXT: %{{.*}} = llvm.intr.vector.extract %[[VEC]][0] : vector<8xf32> from vector<[4]xf32>
   %0 = vector.scalable.extract %vec[0] : vector<8xf32> from vector<[4]xf32>
   return %0 : vector<8xf32>
 }
+
+// -----
+
+// CHECK-LABEL: @make_fixed_vector_of_scalable_vector
+func.func @make_fixed_vector_of_scalable_vector(%f : f64) -> vector<3x[2]xf64>
+{
+  // CHECK: %{{.*}} = llvm.mlir.undef : !llvm.array<3 x vector<[2]xf64>>
+  %res = vector.broadcast %f : f64 to vector<3x[2]xf64>
+  return %res : vector<3x[2]xf64>
+}