cmd/compile: mark temps with new AutoTemp flag, and use it.

This is an extension of
https://go-review.googlesource.com/c/31662/
to mark all the temporaries, not just the ssa-generated ones.

Before-and-after ls -l `go tool -n compile` shows a 3%
reduction in size (or rather, a prior 3% inflation for
failing to filter temps out properly.)

Replaced name-dependent "is it a temp?" tests with calls to
*Node.IsAutoTmp(), which depends on AutoTemp.  Also replace
calls to istemp(n) with n.IsAutoTmp(), to reduce duplication
and clean up function name space.  Generated temporaries
now come with a "." prefix to avoid (apparently harmless)
clashes with legal Go variable names.

Fixes #17644.
Fixes #17240.

Change-Id: If1417f29c79a7275d7303ddf859b51472890fd43
Reviewed-on: https://go-review.googlesource.com/32255
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -72,6 +72,15 @@ type Node struct {
 	flags     uint8 // TODO: store more bool fields in this flag field
 }
 
+// IsAutoTmp indicates if n was created by the compiler as a temporary,
+// based on the setting of the .AutoTemp flag in n's Name.
+func (n *Node) IsAutoTmp() bool {
+	if n == nil || n.Op != ONAME {
+		return false
+	}
+	return n.Name.AutoTemp
+}
+
 const (
 	hasBreak = 1 << iota
 	isClosureVar
@@ -188,6 +197,7 @@ type Name struct {
 	Byval     bool // is the variable captured by value or by reference
 	Needzero  bool // if it contains pointers, needs to be zeroed on function entry
 	Keepalive bool // mark value live across unknown assembly call
+	AutoTemp  bool // is the variable a temporary (implies no dwarf info. reset if escapes to heap)
 }
 
 type Param struct {