cmd/compile: don't mark uintptr->unsafe.Pointer conversion unsafe points
In the past, we planned to implement asynchronous preemption using
precise register pointer maps. In this strategy, conversions between
unsafe.Pointer and uintptr would need to be marked as unsafe points,
as if a pointer value is temporarily converted to uintptr (and not
live otherwise), the GC would not be able to see it when scanning
the stack (and registers).
But now we actually implemented asynchronous preemption with inner
frame conservative scan. So even if a pointer value lives as an
integer the GC can still see it. There is no need to mark the
conversion as unsafe points. This allows more places to be
preempted, as well as for debugger to inject a call.