[AArch64][NEON][SVE] Lower FCOPYSIGN using AArch64ISD::BSP

This patch modifies the FCOPYSIGN lowering to go through the BSP
pseudo-instruction. This allows the same lowering code for NEON,
SVE and SVE2.

As part of this, lowering for BSP for SVE and SVE2 is also added.

For SVE and NEON this patch is NFC.

Differential Revision: https://reviews.llvm.org/D118394