Index: llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/trunk/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -3276,7 +3276,7 @@
             SI->getOperand(1), VectorType::get(ScalarTruncatedTy, Elements1));
 
         NewI = B.CreateShuffleVector(O0, O1, SI->getMask());
-      } else if (isa<LoadInst>(I)) {
+      } else if (isa<LoadInst>(I) || isa<PHINode>(I)) {
         // Don't do anything with the operands, just extend the result.
         continue;
       } else if (auto *IE = dyn_cast<InsertElementInst>(I)) {
@@ -3291,7 +3291,8 @@
             EE->getOperand(0), VectorType::get(ScalarTruncatedTy, Elements));
         NewI = B.CreateExtractElement(O0, EE->getOperand(2));
       } else {
-        llvm_unreachable("Unhandled instruction type!");
+        // If we don't know what to do, be conservative and don't do anything.
+        continue;
       }
 
       // Lastly, extend the result.
Index: llvm/trunk/test/Transforms/LoopVectorize/SystemZ/pr38110.ll
===================================================================
--- llvm/trunk/test/Transforms/LoopVectorize/SystemZ/pr38110.ll
+++ llvm/trunk/test/Transforms/LoopVectorize/SystemZ/pr38110.ll
@@ -0,0 +1,50 @@
+; RUN: opt -passes='loop-vectorize' -mcpu=z13 -force-vector-width=2 -S < %s | FileCheck %s
+;
+; Forcing VF=2 to trigger vector code gen
+;
+; This is a test case to exercise more cases in truncateToMinimalBitWidths().
+; Test passes if vector code is generated w/o hitting llvm_unreachable().
+;
+; Performing minimal check in the output to ensure the loop is actually
+; vectorized.
+;
+; CHECK: vector.body
+
+target datalayout = "E-m:e-i1:8:16-i8:8:16-i64:64-f128:64-v128:64-a:8:16-n32:64"
+target triple = "s390x-ibm-linux"
+
+define void @test(i32 zeroext %width, i8* nocapture %row, i16 zeroext %src, i16* nocapture readonly %dst) {
+entry:
+  %cmp10 = icmp eq i32 %width, 0
+  br i1 %cmp10, label %for.end, label %for.body.lr.ph
+
+for.body.lr.ph:                                   ; preds = %entry
+  %conv1 = zext i16 %src to i32
+  br label %for.body
+
+for.body:                                         ; preds = %for.inc, %for.body.lr.ph
+  %i.012 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.inc ]
+  %sp.011 = phi i8* [ %row, %for.body.lr.ph ], [ %incdec.ptr, %for.inc ]
+  %0 = load i8, i8* %sp.011, align 1
+  %conv = zext i8 %0 to i32
+  %cmp2 = icmp eq i32 %conv, %conv1
+  br i1 %cmp2, label %if.then, label %for.inc
+
+if.then:                                          ; preds = %for.body
+  %1 = load i16, i16* %dst, align 2
+  %conv4 = trunc i16 %1 to i8
+  store i8 %conv4, i8* %sp.011, align 1
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body, %if.then
+  %inc = add nuw i32 %i.012, 1
+  %incdec.ptr = getelementptr inbounds i8, i8* %sp.011, i64 1
+  %exitcond = icmp eq i32 %inc, %width
+  br i1 %exitcond, label %for.end.loopexit, label %for.body
+
+for.end.loopexit:                                 ; preds = %for.inc
+  br label %for.end
+
+for.end:                                          ; preds = %for.end.loopexit, %entry
+  ret void
+}