[llvm] [VectorCombine] Fold insertelement chains of scalar parts to a bitcast and shuffle (PR #226224)

[Tim Besard via llvm-commits]

maleadt wrote:

The `undef`-deprecator failure is expected: `undef_base` and `undef_base_missing_elts` test that the fold only accepts an `undef` base when every lane is overwritten, because leaving lanes `undef` -> `poison` would not be a refinement.

https://github.com/llvm/llvm-project/pull/226224