cmd/compile: better write barrier removal when initializing new objects

When initializing a new object, we're often writing
1) to a location that doesn't have a pointer to a heap object
2) a pointer that doesn't point to a heap object

When both those conditions are true, we can avoid the write barrier.

This CL detects case 1 by looking for writes to known-zeroed
locations.  The results of runtime.newobject are zeroed, and we
perform a simple tracking of which parts of that object are written so
we can determine what part remains zero at each write.

This CL detects case 2 by looking for addresses of globals (including
the types and itabs which are used in interfaces) and for nil pointers.

Makes cmd/go 0.3% smaller. Some particular cases, like the slice
literal in #29573, can get much smaller.

TODO: we can remove actual zero writes also with this mechanism.

Update #29573

Change-Id: Ie74a3533775ea88da0495ba02458391e5db26cb9
Reviewed-on: https://go-review.googlesource.com/c/go/+/156363


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>