Handle UINT64_MAX edge case in switch lowering.

src/IceSwitchLowering.cpp

 //===- subzero/src/IceSwitchLowering.cpp - Switch lowering -----------------==//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 57 matching lines @@
   if (NumCases * 2 <= TotalRange)
     return CaseClusters;
   // Unlikely. Would mean can't store size of jump table.
   if (TotalRange == UINT64_MAX)
     return CaseClusters;
   ++TotalRange;
 
   // Replace everything with a jump table
   InstJumpTable *JumpTable =
       InstJumpTable::create(Func, TotalRange, Inst->getLabelDefault());
-  for (const CaseCluster &Case : CaseClusters)
-    for (uint64_t I = Case.Low; I <= Case.High; ++I)
+  for (const CaseCluster &Case : CaseClusters) {
+    // Case.High could be UINT64_MAX which makes the loop awkward. Unwrap the
+    // last iteration to avoid wrap around problems.
+    for (uint64_t I = Case.Low; I < Case.High; ++I)
       JumpTable->addTarget(I - MinValue, Case.Label);
+    JumpTable->addTarget(Case.High - MinValue, Case.Label);
+  }
 
   CaseClusters.clear();
   CaseClusters.emplace_back(MinValue, MaxValue, JumpTable);
 
   return CaseClusters;
 }
 
 bool CaseCluster::tryAppend(const CaseCluster &New) {
   // Can only append ranges with the same target and are adjacent
   bool CanAppend = this->Label == New.Label && this->High + 1 == New.Low;
   if (CanAppend)
     this->High = New.High;
   return CanAppend;
 }
 
 } // end of namespace Ice