[Codegen] Fail to codegen Call::undef, just like `Call::signed_inte…

…ger_overflow`

See discussion in halide#6866.
It's not obvious if that codepath is ever hit,
let's optimistically assume that it is not.

If this turns out to be not true, we'll have to deal with
a more complicated question of the proper lowering for it,
can it be `poison`, or must it be a `freeze poison`.

src/CodeGen_C.cpp

@@ -2558,6 +2558,8 @@ void CodeGen_C::visit(const Call *op) {
         user_error << "Signed integer overflow occurred during constant-folding. Signed"
                       " integer overflow for int32 and int64 is undefined behavior in"
                       " Halide.\n";
+    } else if (op->is_intrinsic(Call::undef)) {
+        user_error << "undef not eliminated before code generation. Please report this as a Halide bug.\n";
     } else if (op->is_intrinsic(Call::prefetch)) {
         user_assert((op->args.size() == 4) && is_const_one(op->args[2]))
             << "Only prefetch of 1 cache line is supported in C backend.\n";

src/CodeGen_LLVM.cpp

@@ -3227,7 +3227,7 @@ void CodeGen_LLVM::visit(const Call *op) {
                       " integer overflow for int32 and int64 is undefined behavior in"
                       " Halide.\n";
     } else if (op->is_intrinsic(Call::undef)) {
-        value = UndefValue::get(llvm_type_of(op->type));
+        user_error << "undef not eliminated before code generation. Please report this as a Halide bug.\n";
     } else if (op->is_intrinsic(Call::size_of_halide_buffer_t)) {
         llvm::DataLayout d(module.get());
         value = ConstantInt::get(i32_t, (int)d.getTypeAllocSize(halide_buffer_t_type));