[SVE] Fix the FP arithmetic instruction costs for SVE

Several FP instructions (fadd, fsub, etc.) were incorrectly assigned
a higher cost for SVE because they have custom lowering, however we
know they are legal. This patch explicitly assigns a cost of 2 to
these opcodes.

Tests added here:

  Analysis/CostModel/AArch64/arith-fp-sve.ll

Differential Revision: https://reviews.llvm.org/D108993