[RISCV] When expanding CONCAT_VECTORS, don't create INSERT_SUBVECTORS for undef subve...

[RISCV] When expanding CONCAT_VECTORS, don't create INSERT_SUBVECTORS for undef subvectors.

For fixed vectors, the undef will get expanded to an all zeros
build_vector. We don't want that so suppress creating the
insert_subvector.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D117379