[mlir] [VectorOps] Merge VectorReduction/VectorReductionV2 into one Op

Summary:
Paying off some technical debt in VectorOps, where I introduced a special
op for a fused accumulator into reduction to avoid some issues around
printing and parsing an optional accumulator. This CL merges the two
into one op again and does things the right way (still would be nice
to have "assemblyFormat" for optional operands though....).

Reviewers: nicolasvasilache, andydavis1, ftynse, rriddle

Reviewed By: nicolasvasilache

Subscribers: mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, Joonsoo, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75699