[llvm] [GVN] Support rnflow pattern matching and transform (PR #162259)

[Madhur Amilkanthwar via llvm-commits]

madhur13490 wrote:

> AFAIK, there are still a couple of suggestions:
> 
> * no being dependent on the specific condition - the transform cares that we choose one or another index, but not about the specific criteria involved in choosing it, right?

Thanks @momchil for reminding me.  I have updated the patch to 
* made it more generic and that should handle your's and @nikic comments. Summarized the predicate generalisation (filter removed; LHS-matched case canonicalized via swapOperands()
* covered by the two new positive tests

For second suggestion,
> * what if the indices are `i32` or `i64` throughout with no conversion shenanigans?

I would like make that further generalization as a separate patch as the transform is already a big piece. Keeping this patch simple and make the next one incremental will make the work manageable.

Ping to @nikic and @alinas to also take a look.

https://github.com/llvm/llvm-project/pull/162259