perf machine: Optimize a bit the machine__findnew_thread() methods

perf machine: Optimize a bit the machine__findnew_thread() methods

In some cases we already have calculated the hash bucket, so reuse it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Lukasz Odzioba <lukasz.odzioba@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-800zehjsyy03er4s4jf0e99v@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>