Compiler panics when using type aliases and arrays defined using const generics

#7601 introduces a `should_pass/language/const_generics_array_in_reassignments` tests with the following code snippet:

```sway
trait ConstructSelf {
fn construct_self(default: D) -> Self;
}

impl ConstructSelf for [T; N]
where
T: AbiEncode + PartialEq,
{
fn construct_self(default: T) -> Self {
let mut array = [default; N];
let mut i = 0;
while i < N {
array[i] = default;
assert_eq(array[i], default);
i += 1;
}
array
}
}

type ArrayU842 = [u8;42];
```

When calling:

```sway
let array = ArrayU842::construct_self(42u8);
```

the compiler panics with:

```
thread 'main' (2868621) panicked at sway-core/src/ir_generation/const_eval.rs:554:13:
assertion failed: decl.value.is_some()
```

Related to #7616.

Start with the should_pass/language/const_generics_array_in_reassignments test and reproduce the type-alias call using the provided snippet. Then inspect sway-core/src/ir_generation/const_eval.rs at the reported assertion, and verify that the test compiles and runs without a compiler panic after the issue is addressed.

Written by the indexing model from the issue text.