[AArch64][TTI] Cost model FADD/FSUB/FNEG

This lowers the cost for FADD, FSUB, and FNEG. The motivation is to avoid
over-eager SLP vectorisation, that makes it look like SLP vectorisation is
profitable but results in significant slow downs. Lowering the cost for scalar
FADD/FSUB costs helps the profitability decision to favour the scalar
version where vectorisation isn't beneficial.

Lowering the cost for these floating point operations makes sense because a lot
of other instructions including many shuffles have only a cost of 1; these
FADD/FSUB/FNEG instructions should not be twice the cost.

Performance results show a 7% improvement for Imagick from SPEC FP 2017, a
small improvement in Blender, and unchanged results for the other apps in SPEC.
RAJAPerf is neutral and mostly shows no changes.

Differential Revision: https://reviews.llvm.org/D146033