Suggested by degasus.

I can't reproduce that it's actually faster and it will definitely be
slower with position caching for zfreeze.

Address static memory relative to a base register, analog to what we're
doing with PPCSTATE in the CPU JIT. This allows executable memory for
the vertex loader JIT to be allocated anywhere, not just within 2 GiB of
static data.

Fixes issue 8180.

Move ownership of cached_arraybases from CPMemory to VertexLoaderManager
to better match it usage.

Disable bbox

Cleanup OpArg, make immediates more explicit.