Scalarizer: explicitly exclude scalable vectors

They are unsupported and would previously crash, now we just skip them.

Hypothetically, one could consider "scalarizing" a <vscale x n x T> into
n copies of <vscale x 1 x T>. But (1) it's unclear how to do that
because insertelement etc. don't work with scalable vectors in the
required way, and (2) there is no user of such functionality.

Differential Revision: https://reviews.llvm.org/D139358