`torch.aten.mm` fails to legalize with mixed precision operands

@IanWood1 is already working on this.

Since Jan 5, 2026.

(thanks to Claude Sonnet 4.5 for summarizing the issue)

# Title
`torch.aten.mm` fails to legalize with mixed precision operands (f32 + bf16), while `torch.aten.matmul` (batched) works correctly

## Description

There's an inconsistency in type promotion handling between batched and non-batched matrix multiplication operations in torch-mlir. The lowering from Torch to Linalg correctly handles mixed precision operands for `torch.aten.matmul` (batch matmul) by automatically inserting type extension operations, but fails to legalize `torch.aten.mm` (regular matmul) with the same mixed precision scenario.

## Expected Behavior

Both `torch.aten.matmul` and `torch.aten.mm` should support automatic type promotion when operands have different precisions (e.g., f32 and bf16), inserting appropriate `arith.extf` operations to cast operands to the larger type before performing the operation.

## Actual Behavior

### ✅ Works: Batched matmul with mixed precision

The following operation successfully legalizes:

```mlir
%matmul_C_perm = torch.aten.matmul %matrix_a_perm, %matrix_b_perm : !torch.vtensor<[10,64,16],f32>, !torch.vtensor<[10,16,32],bf16> -> !torch.vtensor<[10,64,32],f32>
```

Lowering to Linalg correctly inserts type promotion:

```mlir
%6 = linalg.generic {indexing_maps = [affine_map<(d0, d1, d2) -> (d0, d1, d2)>, affine_map<(d0, d1, d2) -> (d0, d1, d2)>], iterator_types = ["parallel", "parallel", "parallel"]} ins(%transposed_0 : tensor<10x16x32xbf16>) outs(%5 : tensor<10x16x32xf32>) {
^bb0(%in: bf16, %out: f32):
%13 = arith.extf %in : bf16 to f32
linalg.yield %13 : f32
} -> tensor<10x16x32xf32>

...

%9 = linalg.batch_matmul ins(%transposed, %6 : tensor<10x64x16xf32>, tensor<10x16x32xf32>) outs(%8 : tensor<10x64x32xf32>) -> tensor<10x64x32xf32>
```

Note that `linalg.batch_matmul` receives operands of the same type (both f32).

### ❌ Fails: Regular matmul with mixed precision

The following operation fails to legalize:

```mlir
%matmul_C_perm = torch.aten.matmul %matrix_a_perm, %matrix_b_perm : !torch.vtensor<[16,64],f32>, !torch.vtensor<[64,32],bf16> -> !torch.vtensor<[16,32],f32>
```

**Error:**

```
/tmp/.cache/fusilli/benchmark_matmul_b1_m16_n32_k64_transAfalse_transBfalse_biasfalse_atypef32_btypebf16_outtypef32/iree-compile-input.mlir:17:22: error: failed to legalize operation 'torch.aten.mm' that was explicitly marked illegal
%matmul_C_perm = torch.aten.matmul %matrix_a_perm, %matrix_b_perm : !torch.vtensor<[16,64],f32>, !torch.vtensor<[64,32],bf16> -> !torch.vtensor<[16,32],f32>
^
/tmp/.cache/fusilli/benchmark_matmul_b1_m16_n32_k64_transAfalse_transBfalse_biasfalse_atypef32_btypebf16_outtypef32/iree-compile-input.mlir:17:22: note: see current operation: %22 = "torch.aten.mm"(%12, %21) : (!torch.vtensor<[16,64],f32>, !torch.vtensor<[64,32],bf16>) -> !torch.vtensor<[16,32],f32>
```