compiler: treat notinheap types as not being pointers

compiler: treat notinheap types as not being pointers

By definition, a type is marked notinheap doesn't contain any pointers
that the garbage collector cares about, and neither does a pointer to
such a type.  Change the type descriptors to consistently treat such
types as not being pointers, by setting ptrdata to 0 and gcdata to nil.

Change-Id: Id8466555ec493456ff5ff09f1670551414619bd2
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/384118
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>