cmd/compile/internal/gc: remove isforw predicate table (cleanup)

Was only ever filled with one Etype (TFORW) and only used
in one place. Easier to just check for TFORW.

Change-Id: Icc96da3a22b0af1d7e60bc5841c744916c53341e
Reviewed-on: https://go-review.googlesource.com/c/147285Reviewed-by: Martin Möhrmann <moehrmann@google.com>

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

@@ -147,7 +147,6 @@ var asmhdr string
 var simtype [NTYPE]types.EType
 
 var (
-	isforw    [NTYPE]bool
 	isInt     [NTYPE]bool
 	isFloat   [NTYPE]bool
 	isComplex [NTYPE]bool

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

@@ -1137,7 +1137,7 @@ func dtypesym(t *types.Type) *obj.LSym {
 			return lsym
 		}
 		// TODO(mdempsky): Investigate whether this can happen.
-		if isforw[tbase.Etype] {
+		if tbase.Etype == TFORW {
 			return lsym
 		}
 	}

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

@@ -200,8 +200,6 @@ func typeinit() {
 	isComplex[TCOMPLEX64] = true
 	isComplex[TCOMPLEX128] = true
 
-	isforw[TFORW] = true
-
 	// initialize okfor
 	for et := types.EType(0); et < NTYPE; et++ {
 		if isInt[et] || et == TIDEAL {