[InstCombine] Increase coverage of shuffle-gep

There is an uncovered codepath in InstCombineVectorOps, where a new GEP
instruction is created via buildNew. Fix this coverage hole.

Author: artagnon

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -1978,7 +1978,6 @@ static Value *buildNew(Instruction *I, ArrayRef<Value*> NewOps,
                                 DestTy);
     }
     case Instruction::GetElementPtr: {
-      llvm_unreachable("Foo");
       Value *Ptr = NewOps[0];
       ArrayRef<Value*> Idx = NewOps.slice(1);
       return Builder.CreateGEP(cast<GEPOperator>(I)->getSourceElementType(),

llvm/test/Transforms/InstCombine/vec_shuffle.ll

@@ -1,6 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; REQURIES: asserts
-; RUN: not --crash opt %s -passes=instcombine -disable-output
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
 
 declare <2 x half> @llvm.fabs.v2f16(<2 x half>)
 declare <2 x float> @llvm.fabs.v2f32(<2 x float>)
@@ -251,10 +250,9 @@ define <2 x i8> @test13a(i8 %x1, i8 %x2) {
 
 define <1 x ptr> @shuffle_gep(ptr %x1, ptr %x2) {
 ; CHECK-LABEL: @shuffle_gep(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <2 x i8> poison, i8 [[X2:%.*]], i64 0
-; CHECK-NEXT:    [[TMP2:%.*]] = insertelement <2 x i8> [[TMP1]], i8 [[X1:%.*]], i64 1
-; CHECK-NEXT:    [[D:%.*]] = add <2 x i8> [[TMP2]], <i8 7, i8 5>
-; CHECK-NEXT:    ret <2 x i8> [[D]]
+; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <1 x ptr> poison, ptr [[X2:%.*]], i64 0
+; CHECK-NEXT:    [[RET:%.*]] = getelementptr i8, <1 x ptr> [[TMP1]], i64 5
+; CHECK-NEXT:    ret <1 x ptr> [[RET]]
 ;
   %ins.1 = insertelement <2 x ptr> poison, ptr %x1, i32 0
   %ins.2 = insertelement <2 x ptr> %ins.1, ptr %x2, i32 1