[SDAG] fold extract_vector_elt with undef index

This makes the DAG behavior consistent with IR's extractelement after:
rGb32e4664

https://bugs.llvm.org/show_bug.cgi?id=42689

I've tried to maintain test intent for WebAssembly.
The AMDGPU test is trying to test for crashing or other bad behavior,
but I'm not sure if that's possible after this change.