[RISCV] Fix extract_vector_elt on i1 at idx 0 being inverted

It looks like the intention here is to truncate a XLenVT -> i1, in
which case we should be emitting snez instead of sneq if I'm understanding
correctly.

Reviewed By: jacquesguan, frasercrmck

Differential Revision: https://reviews.llvm.org/D149732