bytesN.length fails to compile in the Slang frontend

`.length` on a fixed-bytes value fails MLIR legalization. Legacy and the C++ frontend fold it to N and still evaluate the base.

```solidity
contract C {
    function f(bytes32 x) public pure returns (uint256) { return x.length; }
}
```

Witnesses: `semanticTests/array/fixed_bytes_length_access.sol`, `semanticTests/various/code_length_contract_member.sol`.
