[DebugInfo][CorrelatedValuePropagation] Meta issue: context-dependent rewrites misdescribe source variables outside proven ranges

## Description

This meta issue tracks a group of debug-info bugs in `correlated-propagation`. In each case, the pass uses range or control-flow information to replace an operation with a result that is equivalent wherever that result affects program behavior. The executable semantics are preserved, but a `#dbg_value` describing a source variable is also updated to use the replacement result outside the range where the equivalence is valid.

As a result, the debugger can display a concrete value that the source program never assigned to the variable. The individual reports cover signed-to-unsigned rewrites, elimination of operations, narrowing of operations, and saturating-to-normal rewrites. Each report includes a standalone C reproducer, relevant IR excerpts, and LLDB output before and after `correlated-propagation`.

Although these bugs share a common pattern, each tracked issue is associated with a distinct implementation location in [`llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp`](https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp).

## Common Pattern

The affected transformations generally have the following shape:

```llvm
%source.value =
#dbg_value( %source.value, !variable, ...)
%result = select i1 %condition, %source.value, %fallback
```

`correlated-propagation` proves that a cheaper replacement is equivalent to `%source.value` when `%condition` selects it:

```llvm
%replacement =
#dbg_value( %replacement, !variable, ...)
%result = select i1 %condition, %replacement, %fallback
```

This is correct for `%result`, but the replacement is not necessarily equivalent to the source variable when `%condition` is false. The debug record nevertheless describes the variable with `%replacement`, so a debugger can report a wrong value at a source location where the variable remains inspectable.

## Tracked Issues

### Signed-to-unsigned rewrites

- [ ] #218366
- [ ] #218620
- [ ] #218621
- [ ] #218622
- [ ] #218623
- [ ] #218626
- [ ] #218627

### Operation elimination

- [ ] #218630
- [ ] #218631
- [ ] #218632
- [ ] #218633
- [ ] #219940

### Operation narrowing

- [ ] #218406
- [ ] #218628

### Saturating-to-normal rewrites

- [ ] #220562

Start with llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp, then read the linked reports for their standalone C reproducers, IR excerpts, and LLDB output. This meta issue is complete when the tracked correlated-propagation debug-info bugs are individually addressed and debugger values no longer describe source variables with replacements outside their proven ranges.

Written by the indexing model from the issue text.