x86_64: add behavior test for packed store

Author: xtexx

test/behavior/struct.zig

@@ -2140,3 +2140,25 @@ test "anonymous struct equivalence" {
     comptime assert(A != C);
     comptime assert(B != C);
 }
+
+test "packed struct store" {
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    const S1 = packed struct {
+        val1: u3,
+        val2: u3,
+    };
+    const S2 = packed struct {
+        a: bool,
+        b: bool,
+    };
+
+    var A: S1 = .{ .val1 = 1, .val2 = 1 };
+    A.val2 += 1;
+    try expectEqual(1, A.val1);
+    try expectEqual(2, A.val2);
+    try expect((A.val2 & 1) != 1);
+    const result: S2 = .{ .a = (A.val2 & 1) != 1, .b = (A.val1 & 1) != 1 };
+    try expect(result.a);
+    try expect(!result.b);
+}