[RISCV] Combine (or disjoint ext, ext) -> vwadd (#86929)

DAGCombiner (or InstCombine) will convert an add to an or if the bits
are disjoint, which can prevent what was originally an (add {s,z}ext,
{s,z}ext) from being selected as a vwadd.

This teaches combineBinOp_VLToVWBinOp_VL to recover it by treating it as
an add.