[RISCV] Rename vfredusum/vfredosum intrinsic test files. Merge some tests. NFC

I recently renamed some tests from vfredsum to vfredusum without
noticing they tested both ordered and unordered reductions. This
patch renames them back.

I've also merged test files for vfwredosum/vfwredusum into a single
file for consistency with the other 3 floating point reduction test
files. The inconsistency is what caused my original confusion.