[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