perf evsel: Introduce is_group_member method

To clarify what is being tested, instead of assuming that evsel->leader
== NULL means either an 'isolated' evsel or a 'group leader'.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-lvdbvimaxw9nc5een5vmem0c@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>