[CodeGen] Fix neutral value of vecreduce fadd in tests (NFC)

The neutral value is -0.0, not 0.0. This doesn't matter for "fast"
reductions due to nsz, but does matter for reassoc-only and seq
reductions.

Change tests to mostly use -0.0 where the neutral value was intended,
and add some additional test coverage in some places. Also update
LangRef to use the right value.