[llvm] [DAGCombiner] Restrict mergeTruncStores to pre-type-legalization combines (PR #222245)

[Chung-Yi Chen via llvm-commits]

ADNRs wrote:

To be transparent, I disclose my LLM usage as follows:
1. Polishing code comments
2. Polishing commit messages
3. Polishing PR descriptions
4. Locating the cause of the bug
5. Tracing the commit history of mergeTruncStores() to help me decide this minimal fix
6. Automating my local testing pipeline and reporting the numbers

I am not familiar with the backend, but I have done my best to ensure the fix is reasonable.

https://github.com/llvm/llvm-project/pull/222245