[RISCV] Add TargetConstraintType=2 to vnclip pseudoinstructions. NFC

These instructions are very similar to narrowing shift instructions
which already have this.

Remove TargetConstraintType parameter from VPseudoBinaryV_WV
class. Only 2 was ever passed to it. Pass 2 directly to the classes
instantiated from VPseudoBinaryV_WV instead.