cmd/compile: enforce no uses of Curfn in backend

Updates #15756

Change-Id: Id8d65ca9a3f1a7f9ea43e26cdd5e7d3befef8ba0
Reviewed-on: https://go-review.googlesource.com/38593
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/gen.go

@@ -225,5 +225,7 @@ func tempAt(pos src.XPos, curfn *Node, t *Type) *Node {
 	// TODO(mdempsky/josharian): Remove all reads and writes of lineno and Curfn.
 	lineno = pos
 	Curfn = curfn
-	return temp(t)
+	n := temp(t)
+	Curfn = nil
+	return n
 }

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

@@ -299,6 +299,9 @@ func compile(fn *Node) {
 		return
 	}
 
+	// From this point, there should be no uses of Curfn. Enforce that.
+	Curfn = nil
+
 	// Build an SSA backend function.
 	ssafn := buildssa(fn)
 	if nerrors != 0 {