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/pgen.go

@@ -422,6 +422,9 @@ func compile(fn *Node) {
 			}
 			fallthrough
 		case PPARAM, PPARAMOUT:
+			if n.IsAutoTmp() { // skip debugging info for temporaries
+				continue
+			}
 			p := Gins(obj.ATYPE, n, nil)
 			p.From.Sym = obj.Linklookup(Ctxt, n.Sym.Name, 0)
 			p.To.Type = obj.TYPE_MEM