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

[Han-Kuan Chen via llvm-commits]

HanKuanChen wrote:

No. As you said, SLP assumes that VL consists of InsertElement and comes from the same source. In addition, `getElementIndex` is widely used to vectorize InsertElement, but it does not support PoisonValue.

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