Only emit incoming values for the phi when needed (#37285)

Fixes a crash when one of the incoming value for a union value phi node is of type `Union{}`.
Incoming value of this type can be generated by try-catch.

Fix #37265

Author: yuyichao

src/codegen.cpp

@@ -6836,19 +6836,25 @@ static std::pair<std::unique_ptr<Module>, jl_llvm_functions_t>
             }
             else {
                 Value *RTindex;
-                Value *V;
+                // The branch below is a bit too complex for GCC to realize that
+                // `V` is always initialized when it is used.
+                // Ref https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96629
+                Value *V = nullptr;
                 if (val.typ == (jl_value_t*)jl_bottom_type) {
-                    V = undef_value_for_type(VN->getType());
+                    if (VN)
+                        V = undef_value_for_type(VN->getType());
                     RTindex = UndefValue::get(T_int8);
                 }
                 else if (jl_is_concrete_type(val.typ) || val.constant) {
                     size_t tindex = get_box_tindex((jl_datatype_t*)val.typ, phiType);
                     if (tindex == 0) {
-                        V = boxed(ctx, val);
+                        if (VN)
+                            V = boxed(ctx, val);
                         RTindex = ConstantInt::get(T_int8, 0x80);
                     }
                     else {
-                        V = ConstantPointerNull::get(cast<PointerType>(T_prjlvalue));
+                        if (VN)
+                            V = V_rnull;
                         Type *lty = julia_type_to_llvm(ctx, val.typ);
                         if (dest && !type_is_ghost(lty)) // basically, if !ghost union
                             emit_unbox(ctx, lty, val, val.typ, dest);
@@ -6868,7 +6874,8 @@ static std::pair<std::unique_ptr<Module>, jl_llvm_functions_t>
                         }
                         new_union.TIndex = RTindex;
                     }
-                    V = new_union.Vboxed ? new_union.Vboxed : ConstantPointerNull::get(cast<PointerType>(T_prjlvalue));
+                    if (VN)
+                        V = new_union.Vboxed ? new_union.Vboxed : V_rnull;
                     if (dest) { // basically, if !ghost union
                         Value *skip = NULL;
                         if (new_union.Vboxed != nullptr)

test/core.jl

@@ -7288,3 +7288,35 @@ function f37044(r)
 end
 r37044 = Ref37044(A37044{Int}.body)
 @test f37044(r37044)[1] === Int
+
+a37265() = 0
+b37265() = 0
+function c37265(d)
+    if d == 1
+        e = a37265
+    elseif d == 2
+        e = b37265
+    else
+        try
+        catch
+        end
+    end
+    e
+end
+@test_throws UndefVarError c37265(0)
+@test c37265(1) === a37265
+@test c37265(2) === b37265
+
+function c37265_2(d)
+    if 0
+        e = a37265
+    elseif 0
+        e = b37265
+    else
+        try
+        catch
+        end
+    end
+    e
+end
+@test_throws TypeError c37265_2(0)