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>

src/cmd/compile/internal/gc/sinit.go

@@ -729,6 +729,7 @@ func fixedlit(ctxt initContext, kind initKind, n *Node, var_ *Node, init *Nodes)
 			if r.Sym.IsBlank() {
 				return nblank, r.Left
 			}
+			setlineno(r)
 			return nodSym(ODOT, var_, r.Sym), r.Left
 		}
 	default:
@@ -756,7 +757,7 @@ func fixedlit(ctxt initContext, kind initKind, n *Node, var_ *Node, init *Nodes)
 		}
 
 		// build list of assignments: var[index] = expr
-		setlineno(value)
+		setlineno(a)
 		a = nod(OAS, a, value)
 		a = typecheck(a, ctxStmt)
 		switch kind {