[IR] Change the default value of InstertElement to poison (1/4)

This patch is for fixing potential insertElement-related bugs like D93818.
```
V = UndefValue::get(VecTy);
for(...)
  V = Builder.CreateInsertElementy(V, Elt, Idx);
=>
V = PoisonValue::get(VecTy);
for(...)
  V = Builder.CreateInsertElementy(V, Elt, Idx);
```
Like above, this patch changes the placeholder V to poison.
The patch will be separated into several commits.

Reviewed By: aqjune

Differential Revision: https://reviews.llvm.org/D110311