[dev.ssa] cmd/compile: implement OCHECKNIL

Change-Id: Ifb6b2ad6078ad084b73c0e785e748e92139684e2
Reviewed-on: https://go-review.googlesource.com/14338Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1621,6 +1621,11 @@ func (s *state) expr(n *Node) *ssa.Value {
 	case OADDR:
 		return s.addr(n.Left)
 
+	case OCHECKNIL:
+		p := s.expr(n.Left)
+		s.nilCheck(p)
+		return p
+
 	case OINDREG:
 		if int(n.Reg) != Thearch.REGSP {
 			s.Unimplementedf("OINDREG of non-SP register %s in expr: %v", obj.Rconv(int(n.Reg)), n)