cmd/compile: don't modify nodfp in AllocFrame

nodfp is a global, so modifying it is unsafe in a concurrent backend.
It is also not necessary, since the Used marks
are only relevant for nodes in fn.Dcl.
For good measure, mark nodfp as always used.

Passes toolstash-check.

Updates #15756

Change-Id: I5320459f5eced2898615a17b395a10c1064bcaf5
Reviewed-on: https://go-review.googlesource.com/39200
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -207,7 +207,6 @@ func (s *ssafn) AllocFrame(f *ssa.Func) {
 		if ls, ok := l.(ssa.LocalSlot); ok {
 			ls.N.(*Node).SetUsed(true)
 		}
-
 	}
 
 	scratchUsed := false
@@ -215,7 +214,11 @@ func (s *ssafn) AllocFrame(f *ssa.Func) {
 		for _, v := range b.Values {
 			switch a := v.Aux.(type) {
 			case *ssa.ArgSymbol:
-				a.Node.(*Node).SetUsed(true)
+				n := a.Node.(*Node)
+				// Don't modify nodfp; it is a global.
+				if n != nodfp {
+					n.SetUsed(true)
+				}
 			case *ssa.AutoSymbol:
 				a.Node.(*Node).SetUsed(true)
 			}

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

@@ -463,4 +463,5 @@ func finishUniverse() {
 	nodfp = newname(lookup(".fp"))
 	nodfp.Type = Types[TINT32]
 	nodfp.Class = PPARAM
+	nodfp.SetUsed(true)
 }