Auto merge of #127324 - DianQK:match-br, r=cjgillot

Simplify match based on the cast result of `IntToInt`

Continue to complete #124150. The condition in #120614 is wrong, e.g. `-1i8` cannot be converted to `255i16`. I've rethought the issue and simplified the conditional judgment for a more straightforward approach. The new approach is to check **if the case value after the `IntToInt` conversion equals the target value**.

In different types, `IntToInt` uses different casting methods. This rule is as follows:

- `i8`/`u8` to `i8`/`u8`: do nothing.
- `i8` to `i16`/`u16`: sign extension.
- `u8` to `i16`/`u16`: zero extension.
- `i16`/`u16` to `i8`/`u8`: truncate to the target size.

The previous error was a mix of zext and sext.

r? mir-opt

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