[turbofan] Turn DCHECK for fixed slot index into a CHECK.

This is a temporary workaround to ensure that we crash in release mode
instead of running into undefined behavior.

BUG=chromium:444681
LOG=y
R=ishell@chromium.org

Review URL: https://codereview.chromium.org/800713006

Cr-Commit-Position: refs/heads/master@{#25933}

src/compiler/instruction.h

@@ -129,7 +129,8 @@ class UnallocatedOperand : public InstructionOperand {
     value_ |= VirtualRegisterField::encode(kInvalidVirtualRegister);
     value_ |= BasicPolicyField::encode(policy);
     value_ |= index << FixedSlotIndexField::kShift;
-    DCHECK(this->fixed_slot_index() == index);
+    // TODO(dcarney): 2^10 is not enough for the fixed slot index.
+    CHECK(this->fixed_slot_index() == index);
   }
 
   UnallocatedOperand(ExtendedPolicy policy, int index)