[WASM] Prevent casting `undef` to `CosntantSDNode`

WebAssembly tries to cast an `undef` to `CosntantSDNode` during `LowerAccessVectorElement`.
These operations will trigger an assertion error in cast.
To avoid this issue, we prevent casting, and abort the lowering operation.
A unit test is also included.

This patch fixes [pr61828](https://github.com/llvm/llvm-project/issues/61828)

Reviewed By: tlively

Differential Revision: https://reviews.llvm.org/D147198