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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Given vector_insert always gets expanded in instcombine a couple of passes later and its causing problems with missed folds in vectorcombine - could SLP can just emit shuffles?

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