ICE: Variable slot is too deep inside stack

### What is wrong?

Please include information like:

* full output of the error you received
```
Error: Internal exception in StandardCompiler::compile: /solidity/libyul/backends/evm/EVMObjectCompiler.cpp(67): Throw in function void solidity::yul::EVMObjectCompiler::run(solidity::yul::Object&, bool)
Dynamic exception type: boost::wrapexcept
std::exception::what: Variable return_val is 1 slot(s) too deep inside the stack.
[solidity::util::tag_comment*] = Variable return_val is 1 slot(s) too deep inside the stack.

thread 'main' panicked at 'Yul compilation failed with the above errors', crates/driver/src/lib.rs:90:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```
* what command you ran
```
cargo run --features solc-backend -- --optimize false foo.fe
```

* the code that caused the failure (see [this link](https://help.github.com/articles/basic-writing-and-formatting-syntax/) for help with formatting code)
```
contract TooManyVariables:
pub fn foo(self):
let x0: i32 = 1
let x1: i32 = 1
let x2: i32 = 1
let x3: i32 = 1
let x4: i32 = 1
let x5: i32 = 1
let x6: i32 = 1
let x7: i32 = 1
let x8: i32 = 1
let x9: i32 = 1
let x10: i32 = 1
let x11: i32 = 1
let x12: i32 = 1
let x13: i32 = 1
let x14: i32 = 1
let x15: i32 = 1
```

### How can it be fixed
Ref: https://github.com/ethereum/solidity/issues/5766

I don't have any idea to fix the issue other than implementing the `spill` concept used in register allocation, though I feel implementing it is far beyond the scope of the `Fe`. But at least, it would be better to report this error to users instead of panicking.

Reproduce the failure with `cargo run --features solc-backend -- --optimize false foo.fe`, then inspect `crates/driver/src/lib.rs:90` and the reported `EVMObjectCompiler.cpp(67)` failure path. Done means this StackTooDeepError is reported to the user as a compiler error rather than causing a panic.

Written by the indexing model from the issue text.