[llvm] [VectorCombine] Expand `vector_insert` into shufflevector for earlier cost optimizations (#145512) (PR #146479)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Yes please - @laurenmchin if SLP can remove the vector_insert emission then this patch is less useful, but let's keep it open for now in case there are problems with the SLP approach, I'm sorry for the possibly unnecessary work :( 

https://github.com/llvm/llvm-project/pull/146479