[SCEV] Make scalable size representation more explicit

Represent scalable type sizes using C * vscale, where vscale is
the vscale constant expression. This exposes a bit more information
to SCEV, because the vscale multiplier is explicitly modeled in SCEV
(rather than part of the sizeof expression).

This is mainly intended as an alternative to D143642.

Differential Revision: https://reviews.llvm.org/D144624