cmd/compile/internal: use sparseSet, optimize isSynthetic

changedVars was functionally a set, but couldn't be iterated over
efficiently. In functions with many variables, the wasted iteration was
costly. Use a sparseSet instead.

(*gc.Node).String() is very expensive: it calls Sprintf, which does
reflection, etc, etc. Instead, just look at .Sym.Name, which is all we
care about.

Change-Id: Ib61cd7b5c796e1813b8859135e85da5bfe2ac686
Reviewed-on: https://go-review.googlesource.com/92402
Reviewed-by: David Chase <drchase@google.com>

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

@@ -59,6 +59,11 @@ type Node struct {
 	Etype types.EType // op for OASOP, etype for OTYPE, exclam for export, 6g saved reg, ChanDir for OTCHAN, for OINDEXMAP 1=LHS,0=RHS
 }
 
+func (n *Node) IsSynthetic() bool {
+	name := n.Sym.Name
+	return name[0] == '.' || name[0] == '~'
+}
+
 // 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 {