[clang] [CIR] Upstream extract op for VectorType (PR #138413)

[Amr Hesham via cfe-commits]

AmrDeveloper wrote:

> > > Overall looks good. While here, can you please implement a folder for this operation? It should kick-in if both idx and input vector are constants.
> > 
> > 
> > @bcardosolopes I implement it like this snippet
> > ```
> > OpFoldResult cir::VecExtractOp::fold(FoldAdaptor adaptor) {
> >   const auto vectorAttr =
> >       llvm::dyn_cast_if_present<cir::ConstVectorAttr>(adaptor.getVec());
> >   if (!vectorAttr)
> >     return {};
> > 
> >   const auto indexAttr =
> >       llvm::dyn_cast_if_present<cir::IntAttr>(adaptor.getIndex());
> >   if (!indexAttr)
> >     return {};
> > 
> >   const mlir::ArrayAttr elements = vectorAttr.getElts();
> >   const int64_t index = indexAttr.getSInt();
> >   return elements[index];
> > }
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > But I am thinking, is there a case that codegen will perform extractOp directly from ConstVec, not on load or get_global? I see a similar implementation in MLIR Vector Dialect 🤔 I will try to come up with a test case for testing
> 
> I think no need for dyn_casts here, those gets should already reaturn `cir::ConstVectorAttr` and `cir::IntAttr`.

As far as I understood, they will not always return ConstVecAttr and IntAttr because if an index or a vector is coming from parameters, or other variables, they are not constants in that case, and I am thinking even if the vector is constant, we will use load or get_global when we use it in Extract and will not pass it directly like const int 🤔, not sure if there is a case that will trigger that fold

https://github.com/llvm/llvm-project/pull/138413