[TargetLowering][RISCV] Fixed a scalable vector issue when lowering [s|u]mul.overflow intrinsics

    Fixed the vector type issue that where we used getVectorNumElements()
    should be replaced by getVectorElementCount() when lowering these
    intrinsics.

    This is similar to D94149

Signed-off-by: Eric Tang <tangxingxin1008@gmail.com>

Reviewed By: craig.topper, frasercrmck

Differential Revision: https://reviews.llvm.org/D109809