perf cs-etm: Continuously record last branch

perf cs-etm: Continuously record last branch

Every time synthesize instruction sample, the last branch recording will
be reset.  This is fine if the instruction period is big enough, for
example if use the option '--itrace=i100000', the last branch array is
reset for every sample with 100000 instructions per period; before
generate the next instruction sample, there has the sufficient packets
coming to fill the last branch array.

On the other hand, if set a very small period, the packets will be
significantly reduced between two continuous instruction samples, thus
the last branch array is almost empty for new instruction sample by
frequently resetting.

To allow the last branches to work properly for any instruction periods,
this patch avoids to reset the last branch for every instruction sample
and only reset it when flush the trace data.  The last branches will be
reset only for two cases, one is for trace starting, another case is for
discontinuous trace; other cases can keep recording last branches for
continuous instruction samples.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Robert Walker <robert.walker@arm.com>
Cc: Suzuki Poulouse <suzuki.poulose@arm.com>
Cc: coresight ml <coresight@lists.linaro.org>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lore.kernel.org/lkml/20200219021811.20067-3-leo.yan@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>