Make regalloc2 panic-clean: always return errors when impossible constraints occur

Right now, the allocator can panic if the client provides impossible constraints, such as requiring two different operands to be placed in the same `PReg`.

While this represents a programming error in the client and *shouldn't* arise from an invalid input program, it is always better to bubble up errors; we should reserve `panic!()` and `assert!()` for conditions that can only be violated due to errors in regalloc2 itself.

首先追蹤 allocator 對不可能約束的處理，尤其是在要求兩個運算元共用同一個 PReg 時，並找出涉及的 panic!() 或 assert!() 路徑。沿著 allocator 的公開錯誤回傳路徑處理，讓這些情況成為回傳的錯誤，接著為衝突的 PReg 約束新增覆蓋，並確認 allocation 不再發生 panic。

由索引模型根據 Issue 內容生成。