cmd/compile/internal/types: simplify and optimize PtrDataSize

The current implementation of PtrDataSize checks HasPointers each
call, which could lead to exponential blow-up in handling (admittedly
contrived) deeply nested structs.

To avoid the duplicate recursion, this CL incorporates the HasPointers
logic directly int PtrDataSize, and then re-defines HasPointers as
simply "PtrDataSize(t) > 0".

This CL also tightens up HasPointers/PtrDataSize to only be valid on
actual Go types. Fortunately, there was only one instance where this
wasn't already the case (escape analysis), and that's easily fixed
with an extra check for untyped types.

Change-Id: I0044bf9b558a88333aee2ccb137afb6cb4fea1db
Reviewed-on: https://go-review.googlesource.com/c/go/+/345809


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>