[llvm] [GlobalISel] Refactor extractParts() (PR #75223)

[Amara Emerson via llvm-commits]

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@@ -506,6 +506,47 @@ bool llvm::extractParts(Register Reg, LLT RegTy, LLT MainTy, LLT &LeftoverTy,
     return true;
   }
 
+  // Try to use unmerge for irregular vector split where possible
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aemerson wrote:

I think a small MIR example would be helpful for those reading this code.

https://github.com/llvm/llvm-project/pull/75223