[LSV] Change the default value of InstertElement to poison

This patch is changing the InsertElement's placeholder to poison without changing the LSV's behavior.

Regardless of whether `StoreTy` is FixedVectorType or not, the poison value will be overwritten with a different value.
Therefore, whether the InsertElement's placeholder is poison or undef will not affect the result of the program.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D111005