[AArch64][SVE] Use SVE for VLS fcopysign for wide vectors

Currently fcopysign for VLS vectors lowers through NEON even when the
vector width is wider than a NEON vector, causing bad codegen as the
vectors are split. This patch causes SVE to be used for these vectors
instead, giving much better codegen on wide VLS vectors.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D128642