cmd/compile/internal/gc: remove unused state.placeholder field

gc.state.placeholder was added in 5a6e511c but never used.

Change-Id: I5a621507279d5bb1f3991b7a412d9a63039c464e
Reviewed-on: https://go-review.googlesource.com/38755Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -256,9 +256,6 @@ type state struct {
 	// list of PPARAMOUT (return) variables.
 	returns []*Node
 
-	// A dummy value used during phi construction.
-	placeholder *ssa.Value
-
 	cgoUnsafeArgs bool
 	hasdefer      bool // whether the function contains a defer statement
 }