[RISCV] Optimize undef Even vector in getWideningInterleave. (#88221)

We recently optimized the code when the Odd vector was undef to fix a
poison bug.

There are additional optimizations we can do if the even vector is
undef. With Zvbb, we can use a single vwsll. Without Zvbb, we can use a
vzext.vf2 and a vsll.