[RISCV] Split up structs in buildvec tests. NFC

Some of these tests have multiple vid sequence build_vectors in them, and machine CSE tidies up their output.
But now that small build_vectors are sometimes lowered to scalar `vmv.x.s`s, the output is harder to read with no CSE, e.g. see `buildvec_no_vid_v4i8`.

This patch splits them up into separate functions to address this, and also makes the diff in D157976 more clear since it causes `vmv.x.s` to be lowered to more frequently.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D157974