[LoongArch] Permit auto-vectorization using LSX/LASX with `auto-vec` feature (#78943)

With enough codegen complete, we can now correctly report the size of
vector registers for LSX/LASX, allowing auto vectorization (The
`auto-vec` feature needs to be enabled simultaneously).

As described, the `auto-vec` feature is an experimental one. To ensure
that automatic vectorization is not enabled by default, because the
information provided by the current `TTI` cannot yield additional
benefits for automatic vectorization.