[llvm] [InstCombine] Add contributor guide (PR #79007)

[via llvm-commits]

elhewaty wrote:

we added [this](https://github.com/llvm/llvm-project/issues/69803#issuecomment-1774041896):
> If the simplification creates new instructions, it falls into the InstCombine part. You should find the root of the pattern and look through `InstCombinerImpl::visitXXXInst`.

but I sill find the **look through `InstCombinerImpl::visitXXXInst`** part for the new contributors.

https://github.com/llvm/llvm-project/pull/79007