[RISCV] Use tail agnostic policy for fixed vector vwmacc(u).

[RISCV] Use tail agnostic policy for fixed vector vwmacc(u).

This adds new pseudoinstructions with ForceTailAgnostic set. This
matches what we did for non-widening VMACC. We should move to a
tail policy operand on the pseudos when we expand the intrinsic
interface to include the tail policy.