[SCEV] Add support for GEPs over scalable vectors.

Because we have to use a ConstantExpr at some point, the canonical form
isn't set in stone, but this seems reasonable.

The pretty sizeof(<vscale x 4 x i32>) dumping is a relic of ancient
LLVM; I didn't have to touch that code. :)

Differential Revision: https://reviews.llvm.org/D75887