[RISCV] Don't create LMUL=8 pseudo instructions for ternary widening arithmetic instructions

These instructions produce 2*SEW result so the input can't have
an LMUL=8 or the result would need a non-existant LMUL=16. So
only create pseudos for LMUL up to 4.

Differential Revision: https://reviews.llvm.org/D95189