cmd/compile/internal/s390x: replace 4-byte NOP with a 2-byte NOP on s390x

Added a new instruction, NOPH, with the encoding [0x0700](i.e: bcr 0, 0) and
replace the current 4-byte nop that was encoded using the WORD instruction.

This reduces the size of .text section in go binary by around 17KB and make
generated code easier to read.

Change-Id: I6a756df39e93c4415ea6d038ba4af001b8ccb286
Reviewed-on: https://go-review.googlesource.com/c/go/+/194344Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/asm/internal/asm/testdata/s390x.s

@@ -329,6 +329,9 @@ TEXT main·foo(SB),DUPOK|NOSPLIT,$16-0 // TEXT main.foo(SB), DUPOK|NOSPLIT, $16-
 	TCEB	F5, $8                 // ed5000080010
 	TCDB	F15, $4095             // edf00fff0011
 
+	UNDEF                          // 00000000
+	NOPH                           // 0700
+
 	VL	(R15), V1               // e710f0000006
 	VST	V1, (R15)               // e710f000000e
 	VL	(R15), V31              // e7f0f0000806

src/cmd/compile/internal/s390x/ggen.go

@@ -105,8 +105,5 @@ func zeroAuto(pp *gc.Progs, n *gc.Node) {
 }
 
 func ginsnop(pp *gc.Progs) *obj.Prog {
-	p := pp.Prog(s390x.AWORD)
-	p.From.Type = obj.TYPE_CONST
-	p.From.Offset = 0x47000000 // nop 0
-	return p
+	return pp.Prog(s390x.ANOPH)
 }

src/cmd/internal/obj/s390x/a.out.go

@@ -966,6 +966,8 @@ const (
 	AVMSLOG
 	AVMSLEOG
 
+	ANOPH // NOP
+
 	// binary
 	ABYTE
 	AWORD

src/cmd/internal/obj/s390x/anames.go

@@ -696,6 +696,7 @@ var Anames = []string{
 	"VMSLEG",
 	"VMSLOG",
 	"VMSLEOG",
+	"NOPH",
 	"BYTE",
 	"WORD",
 	"DWORD",

src/cmd/internal/obj/s390x/asmz.go

@@ -314,6 +314,9 @@ var optab = []Optab{
 	// undefined (deliberate illegal instruction)
 	{i: 78, as: obj.AUNDEF},
 
+	// 2 byte no-operation
+	{i: 66, as: ANOPH},
+
 	// vector instructions
 
 	// VRX store
@@ -3394,6 +3397,9 @@ func (c *ctxtz) asmout(p *obj.Prog, asm *[]byte) {
 		d2 := c.regoff(&p.To)
 		zRXE(opcode, uint32(p.From.Reg), 0, 0, uint32(d2), 0, asm)
 
+	case 66:
+		zRR(op_BCR, 0, 0, asm)
+
 	case 67: // fmov $0 freg
 		var opcode uint32
 		switch p.As {