[llvm] [InstCombine] Canonicalize `gep T, (gep i8, base, C1), (Index + C2)` into `gep T, (gep i8, base, C1 + C2 * sizeof(T)), Index` (PR #76177)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw edited https://github.com/llvm/llvm-project/pull/76177