[SVE] Remove calls to isScalable from Hexagon

Reviewers: efriedma, sdesmalen, kparzysz, colinl

Reviewed By: kparzysz

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77757