[Liftoff] Fix redundant register moves

Moving a register to itself is not only unnecessary overhead, it also
breaks invariants in the StackTransferRecipe.

R=ahaas@chromium.org

Bug: v8:6600, chromium:793551
Change-Id: I659fd66b4f2d4564c437ed9fb048322af4299d97
Reviewed-on: https://chromium-review.googlesource.com/819231Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49992}

src/wasm/baseline/liftoff-assembler.cc

@@ -158,7 +158,7 @@ class StackTransferRecipe {
         break;
       case VarState::kRegister:
         DCHECK_EQ(dst.reg_class(), src.reg_class());
-        MoveRegister(dst, src.reg());
+        if (dst != src.reg()) MoveRegister(dst, src.reg());
         break;
       case VarState::kConstant:
         LoadConstant(dst, WasmValue(src.i32_const()));
@@ -167,6 +167,7 @@ class StackTransferRecipe {
   }
 
   void MoveRegister(LiftoffRegister dst, LiftoffRegister src) {
+    DCHECK_NE(dst, src);
     DCHECK(!move_dst_regs.has(dst));
     move_dst_regs.set(dst);
     move_src_regs.set(src);

test/mjsunit/regress/wasm/regress-793551.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+load('test/mjsunit/wasm/wasm-constants.js');
+load('test/mjsunit/wasm/wasm-module-builder.js');
+
+const builder = new WasmModuleBuilder();
+builder.addFunction('test', kSig_i_i)
+    .addBody([
+      // body:
+      kExprGetLocal, 0,      // get_local 0
+      kExprGetLocal, 0,      // get_local 0
+      kExprLoop, kWasmStmt,  // loop
+      kExprBr, 0,            // br depth=0
+      kExprEnd,              // end
+      kExprUnreachable,      // unreachable
+    ])
+    .exportFunc();
+builder.instantiate();