SLPVectorizer: Ignore users that are insertelements we can reschedule them

Patch by Arch D. Robison!
SLPVectorizer: Fix typos

Patch by Arch D. Robison!
SLPVectorizer: Take credit for free extractelement instructions

Extract element instructions that will be removed when vectorzing lower the
cost.

Patch by Arch D. Robison!