[TurboProp] Handle allocation of phis which are never used

In certain situations a phi might not be used by later code, and so
is neither spilled nor has a register allocated to it. Handle this
by removing the incorrect DCHECK.

BUG=chromium:1137979,v8:9684

Change-Id: I702dc05dba22e23dac5c1a366a770f18bac45c52
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2471998
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70536}

src/compiler/backend/mid-tier-register-allocator.cc

@@ -2443,9 +2443,13 @@ void SinglePassRegisterAllocator::AllocatePhi(int virtual_register,
     SpillRegisterForVirtualRegister(virtual_register);
   } else {
     RegisterIndex reg = RegisterForVirtualRegister(virtual_register);
-    DCHECK(reg.is_valid());
+    if (reg.is_valid()) {
+      // If the register is valid, assign it as a phi gap move to be processed
+      // at the successor blocks. If no register or spill slot was used then
+      // the virtual register was never used.
       register_state()->UseForPhiGapMove(reg);
     }
+  }
 }
 
 void SinglePassRegisterAllocator::EnsureRegisterState() {

test/mjsunit/regress/regress-1137979.js

+// Copyright 2020 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.
+
+// Flags: --allow-natives-syntax --turboprop --no-lazy-feedback-allocation
+// Flags: --noanalyze-environment-liveness
+
+function foo() {
+  try {
+    bar();
+  } catch (e) {}
+  for (var i = 0; i < 3; i++) {
+    try {
+      %PrepareFunctionForOptimization(foo);
+      %OptimizeOsr();
+    } catch (e) {}
+  }
+}
+
+foo();
+foo();