cmd/internal/obj: eliminate AMODE

AMODE appears to have been intended to allow
a Prog to switch between 16 (!), 32, or 64 bit x86.
It is unused anywhere in the tree.

Passes toolstash-check -all.

Updates #15756

Change-Id: Ic57b257cfe580f29dad81d97e4193bf3c330c598
Reviewed-on: https://go-review.googlesource.com/38445
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/x86/a.out.go

@@ -746,7 +746,6 @@ const (
 	ARETFQ
 	ASWAPGS
 
-	AMODE
 	ACRC32B
 	ACRC32Q
 	AIMUL3Q

src/cmd/internal/obj/x86/anames.go

@@ -687,7 +687,6 @@ var Anames = []string{
 	"RETFL",
 	"RETFQ",
 	"SWAPGS",
-	"MODE",
 	"CRC32B",
 	"CRC32Q",
 	"IMUL3Q",

src/cmd/internal/obj/x86/obj6.go

@@ -79,17 +79,6 @@ func progedit(ctxt *obj.Link, p *obj.Prog) {
 	}
 	p.Mode = int8(ctxt.Mode)
 
-	switch p.As {
-	case AMODE:
-		if p.From.Type == obj.TYPE_CONST || (p.From.Type == obj.TYPE_MEM && p.From.Reg == REG_NONE) {
-			switch int(p.From.Offset) {
-			case 16, 32, 64:
-				ctxt.Mode = int(p.From.Offset)
-			}
-		}
-		obj.Nopout(p)
-	}
-
 	// Thread-local storage references use the TLS pseudo-register.
 	// As a register, TLS refers to the thread-local storage base, and it
 	// can only be loaded into another register: