[SVE] Make cstfp_pred_ty and cst_pred_ty work with scalable splats

Reviewers: efriedma, lebedev.ri, fhahn, c-rhodes, david-arm

Reviewed By: efriedma, david-arm

Subscribers: tschuett, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83001