The `TTI` hooks are used during vectorization for calculating register
pressure. The default implementation defined wrong value for register
number (all register class are 8 registers).
This patch also defines LoongArch's own register classes.
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.
