[RISCV] Separate slideup/down pseudoinstructions from FMA instructions earlier...

[RISCV] Separate slideup/down pseudoinstructions from FMA instructions earlier in the class hierarchy.

Remove RISCVMaskedPseudo from vslideup/down. I hadn't intended to
include them. I missed that they used the same classes as FMA.
They weren't tested and I don't have a use case yet.

This is also needed as I attempt refactor the classes to improve
D151850.