[SLPVectorizer] Fix crash in isShuffle with scalable vectors

D104809 changed `buildTree_rec` to check for extract element instructions
with scalable types. However, if the extract is extended or truncated,
these changes do not apply and we assert later on in isShuffle(), which
attempts to cast the type of the extract to FixedVectorType.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D110640