Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2095,6 +2095,7 @@
   // the largest legal integer type. We need to be conservative here since
   // x86 generates redundant zero-extenstion instructions if the operand is
   // truncated to i8 or i16.
+  bool TruncCond = false;
   if (DL && BitWidth > NewWidth &&
       NewWidth >= DL->getLargestLegalIntTypeSize()) {
     IntegerType *Ty = IntegerType::get(SI.getContext(), NewWidth);
@@ -2105,6 +2106,7 @@
     for (auto &C : SI.cases())
       static_cast<SwitchInst::CaseIt *>(&C)->setValue(ConstantInt::get(
           SI.getContext(), C.getCaseValue()->getValue().trunc(NewWidth)));
+    TruncCond = true;
   }
 
   if (Instruction *I = dyn_cast<Instruction>(Cond)) {
@@ -2115,8 +2117,11 @@
         for (SwitchInst::CaseIt i = SI.case_begin(), e = SI.case_end();
              i != e; ++i) {
           ConstantInt* CaseVal = i.getCaseValue();
-          Constant* NewCaseVal = ConstantExpr::getSub(cast<Constant>(CaseVal),
-                                                      AddRHS);
+          Constant *LHS = cast<Constant>(
+              TruncCond ? ConstantInt::get(SI.getContext(),
+                                           CaseVal->getValue().sext(BitWidth))
+                        : CaseVal);
+          Constant* NewCaseVal = ConstantExpr::getSub(LHS, AddRHS);
           assert(isa<ConstantInt>(NewCaseVal) &&
                  "Result of expression should be constant");
           i.setValue(cast<ConstantInt>(NewCaseVal));
Index: test/Transforms/InstCombine/narrow-switch.ll
===================================================================
--- test/Transforms/InstCombine/narrow-switch.ll
+++ test/Transforms/InstCombine/narrow-switch.ll
@@ -91,3 +91,33 @@
   %retval.0 = phi i32 [ 24, %sw.default ], [ 123, %sw.bb2 ], [ 213, %sw.bb1 ], [ 231, %entry ]
   ret i32 %retval.0
 }
+
+; Make sure to avoid assertion crashes and use the type before
+; truncation to generate the sub constant expressions that leads
+; to the recomputed condition.
+;
+; CHECK-LABEL: @trunc64to59
+; CHECK: switch i59
+; CHECK: i59 0, label
+; CHECK: i59 18717182647723699, label
+
+define void @trunc64to59(i64 %a) {
+entry:
+  %tmp0 = and i64 %a, 15
+  %tmp1 = mul i64 %tmp0, -6425668444178048401
+  %tmp2 = add i64 %tmp1, 5170979678563097242
+  %tmp3 = mul i64 %tmp2, 1627972535142754813
+  switch i64 %tmp3, label %sw.default [
+    i64 847514119312061490, label %sw.bb1
+    i64 866231301959785189, label %sw.bb2
+  ]
+
+sw.bb1:
+  br label %sw.default
+
+sw.bb2:
+  br label %sw.default
+
+sw.default:
+  ret void
+}