cmd/asm: delete Addr.Reg2

That which cannot happen has not happened.

No immediate changes to Addr or Prog size.

Change-Id: I4cb9315f2c9f5f92eda340bfc4abb46395fa467f
Reviewed-on: https://go-review.googlesource.com/10513Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/asm/internal/asm/parse.go

@@ -311,10 +311,8 @@ func (p *Parser) operand(a *obj.Addr) bool {
 			a.Reg = r1
 			if r2 != 0 {
 				// Form is R1:R2. It is on RHS and the second register
-				// needs to go into the LHS. This is a horrible hack. TODO.
-				// TODO: If we never see this again, can delete Addr.Reg2.
-				panic("cannot happen")
-				a.Reg2 = r2
+				// needs to go into the LHS.
+				panic("cannot happen (Addr.Reg2)")
 			}
 		}
 		// fmt.Printf("REG %s\n", obj.Dconv(&emptyProg, 0, a))

src/cmd/internal/obj/link.go

@@ -146,7 +146,6 @@ import "encoding/binary"
 type Addr struct {
 	Type   int16
 	Reg    int16
-	Reg2   int16 // RHS of register pair. AX:DX (386)
 	Index  int16
 	Scale  int16 // Sometimes holds a register.
 	Name   int8