[llvm] Decompose gep of complex type struct to its element type (PR #107848)

[via llvm-commits]

vfdff wrote:

hi, nikic, thanks for your comment.
1) For the 1st point, can I add  **OneUse** to avoid that issue you mentioned?
2) For the 2nd point, It seems similar to pr96606, so do you mean that we should also driven by **GEP.getSourceElementType()**, but not the **GEP.getResultElementType()** ?

https://github.com/llvm/llvm-project/pull/107848