cmd/compile: keep pointer input arguments live throughout function

Introduce a KeepAlive op which makes sure that its argument is kept
live until the KeepAlive.  Use KeepAlive to mark pointer input
arguments as live after each function call and at each return.

We do this change only for pointer arguments.  Those are the
critical ones to handle because they might have finalizers.
Doing compound arguments (slices, structs, ...) is more complicated
because we would need to track field liveness individually (we do
that for auto variables now, but inputs requires extra trickery).

Turn off the automatic marking of args as live.  That way, when args
are explicitly nulled, plive will know that the original argument is
dead.

The KeepAlive op will be the eventual implementation of
runtime.KeepAlive.

Fixes #15277

Change-Id: I5f223e65d99c9f8342c03fbb1512c4d363e903e5
Reviewed-on: https://go-review.googlesource.com/22365
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -68,11 +68,37 @@ type Node struct {
 	Used      bool
 	Isddd     bool // is the argument variadic
 	Implicit  bool
-	Addrtaken bool // address taken, even if not moved to heap
-	Assigned  bool // is the variable ever assigned to
-	Likely    int8 // likeliness of if statement
-	Hasbreak  bool // has break statement
-	hasVal    int8 // +1 for Val, -1 for Opt, 0 for not yet set
+	Addrtaken bool  // address taken, even if not moved to heap
+	Assigned  bool  // is the variable ever assigned to
+	Likely    int8  // likeliness of if statement
+	hasVal    int8  // +1 for Val, -1 for Opt, 0 for not yet set
+	flags     uint8 // TODO: store more bool fields in this flag field
+}
+
+const (
+	hasBreak = 1 << iota
+	notLiveAtEnd
+)
+
+func (n *Node) HasBreak() bool {
+	return n.flags&hasBreak != 0
+}
+func (n *Node) SetHasBreak(b bool) {
+	if b {
+		n.flags |= hasBreak
+	} else {
+		n.flags &^= hasBreak
+	}
+}
+func (n *Node) NotLiveAtEnd() bool {
+	return n.flags&notLiveAtEnd != 0
+}
+func (n *Node) SetNotLiveAtEnd(b bool) {
+	if b {
+		n.flags |= notLiveAtEnd
+	} else {
+		n.flags &^= notLiveAtEnd
+	}
 }
 
 // Val returns the Val for the node.