[RISCV] Fix the neutral element in vector 'fadd' reductions

Using positive zero as the neutral element in 'fadd' reductions, while
it generates better code, is incorrect. The correct neutral element is
negative zero: 0.0 + -0.0 = 0.0, whereas -0.0 + -0.0 = -0.0.

There are perhaps more optimal lowerings of negative zero avoiding
constant-pool loads which could be left as future work.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D105902