[RISCV] Legalize fixed length (insert_subvector undef, X, 0) to a scalable insert.

[RISCV] Legalize fixed length (insert_subvector undef, X, 0) to a scalable insert.

We were considering this legal, but later the undef would become an all
zeros vector. This would cause us to need to re-legalize the insert later
into a vslideup with zero vector.

This patch catches the case and directly legalizes it to a scalable
insert.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D117377