x64_86 fix: RMI can accept imm16 only for imul r16,r/m16

src/codegen/x86_64/CodeGen.zig

@@ -1850,11 +1850,33 @@ fn asmRegisterMemoryImmediate(
     m: Memory,
     imm: Immediate,
 ) !void {
-    if (switch (imm) {
+    if (reg.size() == .qword and tag[1] == .mul) {
+        // imul does not support imm16 for RMI r64, r/m64
+        const payload = try self.addExtra(Mir.Imm32{ .imm = switch (imm) {
+            .signed => |s| @bitCast(s),
+            .unsigned => |u| @as(u32, @intCast(u)),
+            .nav, .uav, .lazy_sym, .extern_func => unreachable,
+        } });
+        assert(payload + 1 == try self.addExtra(Mir.Memory.encode(m)));
+        _ = try self.addInst(.{
+            .tag = tag[1],
+            .ops = switch (imm) {
+                .signed => .rmi_s,
+                .unsigned => .rmi_u,
+                .nav, .uav, .lazy_sym, .extern_func => unreachable,
+            },
+            .data = .{ .rx = .{
+                .fixes = tag[0],
+                .r1 = reg,
+                .payload = payload,
+            } },
+        });
+    } else if (switch (imm) {
         .signed => |s| if (std.math.cast(i16, s)) |x| @as(u16, @bitCast(x)) else null,
         .unsigned => |u| std.math.cast(u16, u),
         .nav, .uav, .lazy_sym, .extern_func => unreachable,
     }) |small_imm| {
+        // This is only valid for imul RMI r16, r/m16
         _ = try self.addInst(.{
             .tag = tag[1],
             .ops = .rmi,