[solc] Unassigned immutables cryptic error

```
contract C1 {
bool immutable s1 = false;
constructor() { (int8(127) * 3); }
function f() external returns (bool) {
if (s1)
return true;
else
return false;
}
}
```

```
$ solc --experimental-via-ir --optimize test.sol
...
Warning: Source file does not specify required compiler version!
--> test/libsolidity/semanticTests/testYulCompilerOptimisation.sol

Warning: Statement has no effect.
--> test/libsolidity/semanticTests/testYulCompilerOptimisation.sol:3:19:
|
3 | constructor() { (int8(127) * 3); }
| ^^^^^^^^^^^^^^^

Warning: Function state mutability can be restricted to view
--> test/libsolidity/semanticTests/testYulCompilerOptimisation.sol:4:3:
|
4 | function f() external returns (bool) {
| ^ (Relevant source part starts here and spans across multiple lines).

Error: Some immutables were read from but never assigned, possibly because of optimization.
```

Legacy optimizer does not report and error. Legacy and Sol->Yul without optimization don't report either. Both legacy (with and without opt) and Sol->Yul (without opt) result in a runtime failure due to `int8` overflow inside the constructor.

Reproduce the contract with solc --experimental-via-ir --optimize and compare legacy, Sol-to-Yul, and optimization modes. Start with the immutable-read handling and the semantic test path mentioned in test/libsolidity/semanticTests/testYulCompilerOptimisation.sol; clarify the expected diagnostic and cover the agreed behavior with a regression test.

Written by the indexing model from the issue text.