cmd/compile: simplify AMD64's assembly generator

AMD64's ADDQconstmodify/ADDLconstmodify have similar logic with
other constmodify like operators, but seperated case statements.
This CL simplify them with a fallthrough.

Change-Id: Ia73ffeaddc5080182f68c06c9d9b48fe32a14e38
Reviewed-on: https://go-review.googlesource.com/135855
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/amd64/ssa.go

@@ -759,14 +759,9 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 			p.To.Type = obj.TYPE_MEM
 			p.To.Reg = v.Args[0].Reg()
 			gc.AddAux2(&p.To, v, off)
-		} else {
-			p := s.Prog(v.Op.Asm())
-			p.From.Type = obj.TYPE_CONST
-			p.From.Offset = val
-			p.To.Type = obj.TYPE_MEM
-			p.To.Reg = v.Args[0].Reg()
-			gc.AddAux2(&p.To, v, off)
+			break
 		}
+		fallthrough
 	case ssa.OpAMD64ANDQconstmodify, ssa.OpAMD64ANDLconstmodify, ssa.OpAMD64ORQconstmodify, ssa.OpAMD64ORLconstmodify,
 		ssa.OpAMD64XORQconstmodify, ssa.OpAMD64XORLconstmodify:
 		sc := v.AuxValAndOff()