[FuncSpec][NFC] Improve the unittest coverage for constant folding of GEPs.

[labrinea]

Summary

The InstCostVisitor is currently using the DataLayout-independent constant folding interface.
This is a workaround since we can't directly call ConstantExpr::getGetElementPtr due to deprecation.
This patch shows that the constant folding interface we are using is not good enough.

llvm/unittests/Transforms/IPO/FunctionSpecializationTest.cpp

Ref = getInstCost(Sub) + getInstCost(BrBB1) + getInstCost(Sdiv) +
         getInstCost(BrLoop);
   Bonus = Specializer.getSpecializationBonus(F->getArg(2), False, Visitor);
   EXPECT_EQ(Bonus, Ref);
   EXPECT_TRUE(Bonus > 0);
 }
 
 TEST_F(FunctionSpecializationTest, Misc) {
   const char *ModuleString = R"(
-    @g = constant [2 x i32] zeroinitializer, align 4
+    %struct_t = type { [8 x i16], [8 x i16], i32, i32, i32, ptr, [8 x i8] }
+    @g = constant %struct_t zeroinitializer, align 16
 
     declare i32 @llvm.smax.i32(i32, i32)
     declare i32 @bar(i32)
 
     define i32 @foo(i8 %a, i1 %cond, ptr %b, i32 %c) {
       %cmp = icmp eq i8 %a, 10
-      %ext = zext i1 %cmp to i32
-      %sel = select i1 %cond, i32 %ext, i32 1
-      %gep = getelementptr i32, ptr %b, i32 %sel
+      %ext = zext i1 %cmp to i64
+      %sel = select i1 %cond, i64 %ext, i64 1
+      %gep = getelementptr inbounds %struct_t, ptr %b, i64 %sel, i32 4
       %ld = load i32, ptr %gep
       %fr = freeze i32 %ld
       %smax = call i32 @llvm.smax.i32(i32 %fr, i32 1)
       %call = call i32 @bar(i32 %smax)
       %fr2 = freeze i32 %c
       %add = add i32 %call, %fr2
       ret i32 %add
     }
TEST_F(FunctionSpecializationTest, Misc) {
   Bonus = Specializer.getSpecializationBonus(F->getArg(1), True, Visitor);
   EXPECT_EQ(Bonus, Ref);
   EXPECT_TRUE(Bonus > 0);
 
   // gep + load + freeze + smax
   Ref = getInstCost(Gep) + getInstCost(Load) + getInstCost(Freeze) +
         getInstCost(Smax);
   Bonus = Specializer.getSpecializationBonus(F->getArg(2), GV, Visitor);
-  EXPECT_EQ(Bonus, Ref);
-  EXPECT_TRUE(Bonus > 0);
+  EXPECT_NE(Bonus, Ref);
+  EXPECT_FALSE(Bonus > 0);
 
   Bonus = Specializer.getSpecializationBonus(F->getArg(3), Undef, Visitor);
   EXPECT_TRUE(Bonus == 0);
 }