[turbofan] Remove case_count >= 3 limit for switch.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1460583003

Cr-Commit-Position: refs/heads/master@{#32082}

src/compiler/common-operator.cc

@@ -462,7 +462,6 @@ const Operator* CommonOperatorBuilder::IfException(IfExceptionHint hint) {
 
 
 const Operator* CommonOperatorBuilder::Switch(size_t control_output_count) {
-  DCHECK_GE(control_output_count, 3u);        // Disallow trivial switches.
   return new (zone()) Operator(               // --
       IrOpcode::kSwitch, Operator::kKontrol,  // opcode
       "Switch",                               // name

src/compiler/instruction-selector.cc

@@ -593,7 +593,6 @@ void InstructionSelector::VisitControl(BasicBlock* block) {
       DCHECK_EQ(IrOpcode::kIfDefault, sw.default_branch->front()->opcode());
       // All other successors must be cases.
       sw.case_count = block->SuccessorCount() - 1;
-      DCHECK_LE(1u, sw.case_count);
       sw.case_branches = &block->successors().front();
       // Determine case values and their min/max.
       sw.case_values = zone()->NewArray<int32_t>(sw.case_count);

src/compiler/verifier.cc

@@ -250,12 +250,12 @@ void Verifier::Visitor::Check(Node* node) {
             break;
           }
           default: {
-            UNREACHABLE();
+            V8_Fatal(__FILE__, __LINE__, "Switch #%d illegally used by #%d:%s",
+                     node->id(), use->id(), use->op()->mnemonic());
             break;
           }
         }
       }
-      CHECK_LE(1, count_case);
       CHECK_EQ(1, count_default);
       CHECK_EQ(node->op()->ControlOutputCount(), count_case + count_default);
       // Type is empty.