[llvm] [CodeGenPrepare] Fix signed overflow (PR #141487)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> I have a small test locally that triggers the issue with sanitizers enabled (the sanitizer will complain about the addition). But without it enabled I'm not sure how I could test for it.

Does the overflow affect codegen? Like, when it overflows `TLI.isLegalAddressingMode` becomes true (or the other way around). If that's the case you can write a codegen test to demonstrate it.

Also, please refrain from force-push unless it's necessary.

https://github.com/llvm/llvm-project/pull/141487