cmd/compile: simplify Neg lowering on s390x

No need to sign extend input to Neg8 and Neg16.

Change-Id: I7896c83c9cdf84a34098582351a4aabf61cd6fdd
Reviewed-on: https://go-review.googlesource.com/102675
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/gen/S390X.rules

@@ -59,9 +59,7 @@
 (Xor(32|16|8) x y) -> (XORW x y)
 
 (Neg64 x) -> (NEG x)
-(Neg32  x) -> (NEGW x)
-(Neg16  x) -> (NEGW (MOVHreg x))
-(Neg8   x) -> (NEGW (MOVBreg x))
+(Neg(32|16|8) x) -> (NEGW x)
 (Neg32F x) -> (FNEGS x)
 (Neg64F x) -> (FNEG x)
 

src/cmd/compile/internal/ssa/rewriteS390X.go

@@ -4564,19 +4564,13 @@ func rewriteValueS390X_OpMul8_0(v *Value) bool {
 	}
 }
 func rewriteValueS390X_OpNeg16_0(v *Value) bool {
-	b := v.Block
-	_ = b
-	typ := &b.Func.Config.Types
-	_ = typ
 	// match: (Neg16 x)
 	// cond:
-	// result: (NEGW (MOVHreg x))
+	// result: (NEGW x)
 	for {
 		x := v.Args[0]
 		v.reset(OpS390XNEGW)
-		v0 := b.NewValue0(v.Pos, OpS390XMOVHreg, typ.Int64)
-		v0.AddArg(x)
-		v.AddArg(v0)
+		v.AddArg(x)
 		return true
 	}
 }
@@ -4625,19 +4619,13 @@ func rewriteValueS390X_OpNeg64F_0(v *Value) bool {
 	}
 }
 func rewriteValueS390X_OpNeg8_0(v *Value) bool {
-	b := v.Block
-	_ = b
-	typ := &b.Func.Config.Types
-	_ = typ
 	// match: (Neg8 x)
 	// cond:
-	// result: (NEGW (MOVBreg x))
+	// result: (NEGW x)
 	for {
 		x := v.Args[0]
 		v.reset(OpS390XNEGW)
-		v0 := b.NewValue0(v.Pos, OpS390XMOVBreg, typ.Int64)
-		v0.AddArg(x)
-		v.AddArg(v0)
+		v.AddArg(x)
 		return true
 	}
 }