Fix codegen error for some union initializers

Closes #3377

Author: LemonBoy

src/codegen.cpp

@@ -7195,7 +7195,7 @@ check: switch (const_val->special) {
                     union_value_ref = LLVMGetUndef(union_type_ref);
                     make_unnamed_struct = false;
                 } else {
-                    uint64_t field_type_bytes = LLVMStoreSizeOfType(g->target_data_ref,
+                    uint64_t field_type_bytes = LLVMABISizeOfType(g->target_data_ref,
                             get_llvm_type(g, payload_value->type));
                     uint64_t pad_bytes = type_entry->data.unionation.union_abi_size - field_type_bytes;
                     LLVMValueRef correctly_typed_value = gen_const_val(g, payload_value, "");
@@ -7235,7 +7235,7 @@ check: switch (const_val->special) {
                     uint64_t last_field_offset = LLVMOffsetOfElement(g->target_data_ref, LLVMTypeOf(result), 1);
                     uint64_t end_offset = last_field_offset +
                         LLVMStoreSizeOfType(g->target_data_ref, LLVMTypeOf(fields[1]));
-                    uint64_t expected_sz = LLVMStoreSizeOfType(g->target_data_ref, get_llvm_type(g, type_entry));
+                    uint64_t expected_sz = LLVMABISizeOfType(g->target_data_ref, get_llvm_type(g, type_entry));
                     unsigned pad_sz = expected_sz - end_offset;
                     if (pad_sz != 0) {
                         fields[2] = LLVMGetUndef(LLVMArrayType(LLVMInt8Type(), pad_sz));

test/stage1/behavior/union.zig

@@ -620,3 +620,12 @@ test "0-sized extern union definition" {
 
     expect(U.f == 1);
 }
+
+test "union initializer generates padding only if needed" {
+    const U = union(enum) {
+        A: u24,
+    };
+
+    var v = U{ .A = 532 };
+    expect(v.A == 532);
+}