[RISCV] Replace two calls to getMinRVVVectorSizeInBits with getRealMinVLen [nfc]

This doesn't change behavior, it just makes it slightly more obvious what's
going on.  Note that getRealMinVLen is always >= getMinRVVVectorSizeInBits.

The first case is a bit tricky, as you have to know that
getMinRVVVectorSizeInBits returns 0 when not set, and thus is equivalent
to the else value clause.  The new code structure makes it more obvious we
return 0 unless using RVV for fixed length vectors.