[llvm] [InstCombine] Canonicalise SextADD + GEP (PR #69581)

[via llvm-commits]

LiqinWeng wrote:

> Please, can you take a bit more care? Your proof is very obviously broken if you just look at the result.
> 
> Here is a fixed version: https://alive2.llvm.org/ce/z/FSNnoo Note that the `nsw` on the `add` is required for correctness, which you don't check.

Pls see: https://alive2.llvm.org/ce/z/vkCK92. The transformation only occurs when the second operand of the add is a constant.
I will add the `nsw` on the `add` 

https://github.com/llvm/llvm-project/pull/69581