[dev.ssa] cmd/compile: fix build for old backend on ARM64

Apparently the old backend needs NEG instruction having RegRead set,
even this instruction does not take a Reg field... I don't think SSA
uses this flag, so just leave it as it was. SSA is still happy.

Fix ARM64 build on https://build.golang.org/?branch=dev.ssa

Change-Id: Ia7e7f2ca217ddae9af314d346af5406bbafb68e8
Reviewed-on: https://go-review.googlesource.com/25302Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/arm64/prog.go

@@ -44,7 +44,7 @@ var progtable = [arm64.ALAST & obj.AMask]obj.ProgInfo{
 	// Integer
 	arm64.AADD & obj.AMask:   {Flags: gc.SizeQ | gc.LeftRead | gc.RegRead | gc.RightWrite},
 	arm64.ASUB & obj.AMask:   {Flags: gc.SizeQ | gc.LeftRead | gc.RegRead | gc.RightWrite},
-	arm64.ANEG & obj.AMask:   {Flags: gc.SizeQ | gc.LeftRead | gc.RightWrite},
+	arm64.ANEG & obj.AMask:   {Flags: gc.SizeQ | gc.LeftRead | gc.RegRead | gc.RightWrite}, // why RegRead? revisit once the old backend gone
 	arm64.AAND & obj.AMask:   {Flags: gc.SizeQ | gc.LeftRead | gc.RegRead | gc.RightWrite},
 	arm64.AORR & obj.AMask:   {Flags: gc.SizeQ | gc.LeftRead | gc.RegRead | gc.RightWrite},
 	arm64.AEOR & obj.AMask:   {Flags: gc.SizeQ | gc.LeftRead | gc.RegRead | gc.RightWrite},