[RISCV] Support extract_vector_elt for fixed and scalable masked registers.

This uses a really simple approach of converting to an i8 vector
and extracting. This is probably not the best approach especially
if you know the index is constant.

Other ideas:
-Store to stack temporary using vse1, load as scalar and shift.
-Sort of bitcast the vector to a vector of i8, slide down the
 appropriate 8 bit element, copy to scalar, shift down the
 correct bit within the 8 bits we extracted. Not exactly sure
 how to describe such a bitcast from i1 vector to i8 vector
 within the type system for elements less than 8.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D98310