[RISCV] Add a policy operand to VPseudoBinaryNoMaskTU [NFC]

This change adds a policy operand to the helper class which is used for binary ops like vadd, but also, possibly surprisingly, some of the vslide variants. This allows us to represent the tail agnostic state with this pseudo family - previously, we could only represent tail undefined and tail undisturbed. (Since these don't have a mask, they're always mask undefined.)

This is NFC because no current producer uses the tail agnostic state. This will change in an upcoming change to doPeepholeMaskedRVV.

Differential Revision: https://reviews.llvm.org/D153067