[LSR][SCEVExpander] Avoid blind cast 'Factor' to SCEVConstant in FactorOutConstant.

Summary:
In SCEVExpander FactorOutConstant(), when GEP indexing into/over scalable vector,
it is legal for the 'Factor' in a MulExpr to be the size of a scalable vector
instead of a compile-time constant.

Current upstream crash with the test attached.

Reviewers: efriedma, sdesmalen, sanjoy.google, mkazantsev

Reviewed By: efriedma

Subscribers: hiraditya, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80973