perf: update buttongroup style

components/button/style/mixin.less

@@ -271,10 +271,10 @@
   > span:first-child > .@{btnClassName} {
     margin-left: 0;
   }
-  > .@{btnClassName}:first-child:last-child {
+  > .@{btnClassName}:only-child {
     border-radius: @btn-border-radius-base;
   }
-  > span:first-child:last-child > .@{btnClassName} {
+  > span:only-child > .@{btnClassName} {
     border-radius: @btn-border-radius-base;
   }
 
@@ -291,10 +291,10 @@
   }
 
   &-sm {
-    > .@{btnClassName}:first-child:last-child {
+    > .@{btnClassName}:only-child {
       border-radius: @btn-border-radius-sm;
     }
-    > span:first-child:last-child > .@{btnClassName} {
+    > span:only-child > .@{btnClassName} {
       border-radius: @btn-border-radius-sm;
     }
     > .@{btnClassName}:first-child:not(:last-child),