[dev.ssa] cmd/compile: remove dead amd64 ITab lowering rule

ITab is handled by decomposition.
The rule is vestigial. Remove it.

Change-Id: I6fdf3d14d466761c7665c7ea14f34ca0e1e3e646
Reviewed-on: https://go-review.googlesource.com/24718
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -436,7 +436,6 @@
 (GetG mem) -> (LoweredGetG mem)
 (GetClosurePtr) -> (LoweredGetClosurePtr)
 (Addr {sym} base) -> (LEAQ {sym} base)
-(ITab (Load ptr mem)) -> (MOVQload ptr mem)
 
 // block rewrites
 (If (SETL  cmp) yes no) -> (LT  cmp yes no)

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -232,8 +232,6 @@ func rewriteValueAMD64(v *Value, config *Config) bool {
 		return rewriteValueAMD64_OpHmul8(v, config)
 	case OpHmul8u:
 		return rewriteValueAMD64_OpHmul8u(v, config)
-	case OpITab:
-		return rewriteValueAMD64_OpITab(v, config)
 	case OpInterCall:
 		return rewriteValueAMD64_OpInterCall(v, config)
 	case OpIsInBounds:
@@ -4057,26 +4055,6 @@ func rewriteValueAMD64_OpHmul8u(v *Value, config *Config) bool {
 		return true
 	}
 }
-func rewriteValueAMD64_OpITab(v *Value, config *Config) bool {
-	b := v.Block
-	_ = b
-	// match: (ITab (Load ptr mem))
-	// cond:
-	// result: (MOVQload ptr mem)
-	for {
-		v_0 := v.Args[0]
-		if v_0.Op != OpLoad {
-			break
-		}
-		ptr := v_0.Args[0]
-		mem := v_0.Args[1]
-		v.reset(OpAMD64MOVQload)
-		v.AddArg(ptr)
-		v.AddArg(mem)
-		return true
-	}
-	return false
-}
 func rewriteValueAMD64_OpInterCall(v *Value, config *Config) bool {
 	b := v.Block
 	_ = b