`cacheVariable` sometimes is triggered for immutable variables

[CodeSandwich]

For example:

contract ImmutableCached {
    uint256 public immutable one = 1;
    function two() public returns (uint256 result) {
        result = one + one;
    }
}

yields:

### <a name="GAS-1"></a>[GAS-1] State variables should be cached in stack variables rather than re-reading them from storage
The instances below point to the second+ access of a state variable within a function. Caching of a state variable replaces each Gwarmaccess (100 gas) with a much cheaper stack read. Other less obvious fixes/optimizations include having local memory caches of state variable structs, or having local caches of state variable contracts/addresses.

*Saves 100 gas per instance*

*Instances (1)*:
``solidity
File: lol.sol

7:         result = one + one;

``

This is a weird issue, because depending on how code is expressed it may not trigger, e.g. changing

result = one + one;

into

return one + one;

fixes the warning.

[Picodes]

Thanks @CodeSandwich!