[llvm] [RISCV] Always combine scalarized shufflevectors back to vector_shuffle (PR #88147)

[Luke Lau via llvm-commits]

lukel97 wrote:

> > Because the mask is a different length from the source vectors, SelectionDAGBuilder emits this as a series of extract_vector_elts and a build_vector (since vector_shuffle requires the mask and source types be the same).
> 
> 
> 
> That's an over simplification of what SelectionDAGBuilder does. It does handle some cases where the mask is smaller. What it does depends on the indices used.

Yes, this is specifically dealing with single source shuffles where the indices span most of the source

https://github.com/llvm/llvm-project/pull/88147