[RISCV] Remove mask size restriction on single source and dual src shuffle costing (try 2)

Some callers pass in an empty mask to represent "unknown".  We should use the generic costs for these cases.  We can add VL=1 costing seperately if desired.

Reapplying after revert.  A new test had been added, and I'd missed updating it when rebasing before.  This is a great happy accident as I hadn't figured out how to get SLP to exercise this case, I'd merely noticed it via inspection.