cmd/compile: optimize 386's assembly generator

The ADDconstmodify has similar logic with other constmodify like
instructions. This CL optimize them to share code via fallthrough.
And the size of pkg/linux_386/cmd/compile/internal/x86.a decreases
about 0.3KB.

Change-Id: Ibdf06228afde875e8fe8e30851b50ca2be513dd9
Reviewed-on: https://go-review.googlesource.com/136398
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/x86/ssa.go

@@ -547,22 +547,22 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		p.To.Reg = v.Args[0].Reg()
 		gc.AddAux(&p.To, v)
 	case ssa.Op386ADDLconstmodify:
-		var p *obj.Prog = nil
 		sc := v.AuxValAndOff()
-		off := sc.Off()
 		val := sc.Val()
+		if val == 1 || val == -1 {
+			var p *obj.Prog
 			if val == 1 {
 				p = s.Prog(x86.AINCL)
-		} else if val == -1 {
-			p = s.Prog(x86.ADECL)
 			} else {
-			p = s.Prog(v.Op.Asm())
-			p.From.Type = obj.TYPE_CONST
-			p.From.Offset = val
+				p = s.Prog(x86.ADECL)
 			}
+			off := sc.Off()
 			p.To.Type = obj.TYPE_MEM
 			p.To.Reg = v.Args[0].Reg()
 			gc.AddAux2(&p.To, v, off)
+			break
+		}
+		fallthrough
 	case ssa.Op386ANDLconstmodify, ssa.Op386ORLconstmodify, ssa.Op386XORLconstmodify:
 		sc := v.AuxValAndOff()
 		off := sc.Off()