[SVE] Bail from cstfp_pred_ty before walking scalable vector

Summary:
Bail out in cstfp_pred_ty before calling getNumElements and walking the
vector if the vector is scalable.

Reviewers: efriedma, lebedev.ri, fhahn, sdesmalen, fpetrogalli

Reviewed By: sdesmalen

Subscribers: tschuett, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81617