cmd/internal/obj/x86: remove redundant code (fix TODO)

Special cases described in TODO comments are
fixed in https://golang.org/cl/6901.

One of those blocks was needed until old 6a assembler was removed.
This happened in https://golang.org/cl/12784.

Fixes #24734 (Also contains more details and reasoning)

Change-Id: If1f2f155d36ab236b16ae6f309a0716e00aa6371
Reviewed-on: https://go-review.googlesource.com/105156
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/x86/asm6.go

@@ -3739,7 +3739,7 @@ func regIndex(r int16) int {
 // Reports errors via ctxt.
 func avx2gatherValid(ctxt *obj.Link, p *obj.Prog) bool {
 	// If any pair of the index, mask, or destination registers
-	// are the same, this instruction results a #UD fault.
+	// are the same, illegal instruction trap (#UD) is triggered.
 	index := regIndex(p.GetFrom3().Index)
 	mask := regIndex(p.From.Reg)
 	dest := regIndex(p.To.Reg)
@@ -3768,38 +3768,9 @@ func (ab *AsmBuf) doasm(ctxt *obj.Link, cursym *obj.LSym, p *obj.Prog) {
 		ab.Put1(byte(pre))
 	}
 
-	// TODO(rsc): This special case is for SHRQ $3, AX:DX,
-	// which encodes as SHRQ $32(DX*0), AX.
-	// Similarly SHRQ CX, AX:DX is really SHRQ CX(DX*0), AX.
-	// Change encoding generated by assemblers and compilers and remove.
-	if (p.From.Type == obj.TYPE_CONST || p.From.Type == obj.TYPE_REG) && p.From.Index != REG_NONE && p.From.Scale == 0 {
-		p.SetFrom3(obj.Addr{
-			Type: obj.TYPE_REG,
-			Reg:  p.From.Index,
-		})
-		p.From.Index = 0
-	}
-
-	// TODO(rsc): This special case is for PINSRQ etc, CMPSD etc.
-	// Change encoding generated by assemblers and compilers (if any) and remove.
+	// Сhecks to warn about instruction/arguments combinations that
+	// will unconditionally trigger illegal instruction trap (#UD).
 	switch p.As {
-	case AIMUL3Q, APEXTRW, APINSRW, APINSRD, APINSRQ, APSHUFHW, APSHUFL, APSHUFW, ASHUFPD, ASHUFPS, AAESKEYGENASSIST, APSHUFD, APCLMULQDQ:
-		if p.From3Type() == obj.TYPE_NONE {
-			p.SetFrom3(p.From)
-			p.From = obj.Addr{}
-			p.From.Type = obj.TYPE_CONST
-			p.From.Offset = p.To.Offset
-			p.To.Offset = 0
-		}
-	case ACMPSD, ACMPSS, ACMPPS, ACMPPD:
-		if p.From3Type() == obj.TYPE_NONE {
-			p.SetFrom3(p.To)
-			p.To = obj.Addr{}
-			p.To.Type = obj.TYPE_CONST
-			p.To.Offset = p.GetFrom3().Offset
-			p.GetFrom3().Offset = 0
-		}
-
 	case AVGATHERDPD,
 		AVGATHERQPD,
 		AVGATHERDPS,