cmd/compile: disable typPtr caching in the backend
The only new Types that the backend introduces
are pointers to Types generated by the frontend.
Usually, when we generate a *T,
we cache the resulting Type in T,
to avoid recreating it later.
However, that caching is not concurrency safe.
Rather than add mutexes, this CL disables that
caching before starting the backend.
The backend generates few enough new *Ts that the
performance impact of this is small, particularly
if we pre-create some commonly used *Ts.