fadd reduction with
1. Fast flag set
2. No of elements in input vector is power of 2 results in series of
faddp instructions. faddp instruction has latency/throughput identical
to fadd instruction and hence, we set relative cost=1 for faddp as well.
The change didn't show any regression with SPEC17-FP(C/C++),
llvm-test-suite on Neoverse-V2.
c5672e21ca