[RISCV] Prevent crashes when bitcasting between fixed vectors and scalars.

[RISCV] Prevent crashes when bitcasting between fixed vectors and scalars.

Not all scalar element types are allowed in vectors so we may not
be able to bitcast to a 1 element vector to use insert/extract.

This will become a bigger issue when the Zve extensions are commited.
For now, I'm using the ELEN limit to limit the element types.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D113219