[llvm] [WebAssembly] Avoid scanning unrelated debug values (NFCI) (PR #218378)

[Heejin Ahn via llvm-commits]

https://github.com/aheejin commented:

I think this is a clever idea, assuming `DBG_VALUE`s for the same register can be also above the def, whether that's a stray debug value or a debug value attached to a previous def of the same register. Did you notice speedups from your program with this?

https://github.com/llvm/llvm-project/pull/218378