[llvm] [SLP][REVEC] Fix insertelement legality checks (PR #146921)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

Hi @HanKuanChen , thanks for the review! Should I try and apply your suggestion? So far I just created a `Gather` node to be safe, as I don't completely understand the implications of vectorising `insertelement` instructions where some values in `VL` are poison.

https://github.com/llvm/llvm-project/pull/146921