[RISCV] Combine VPseudoUnaryMask and VPseudoUnaryMaskTA [NFC]

The only difference between these classes was the existance of a policy operand on the later.  We can use the policy operand version for the one place which used the non-TA suffixed one.  I then renamed to remove TA as these aren't tail agnostic; they take their policy from the operand.

Note that this wouldn't be strictly NFC except that the one user of the class being removed wasn't in the masked psuedo table, and thus doesn't go through mask to unmasked conversion in ISEL.  That's a missed optimization we may want to fix at some point.