perf script: Enhance sample flags for trace begin / end

perf script: Enhance sample flags for trace begin / end

Allow for different combinations of sample flags with "trace begin" or
"trace end".

Previously, the Intel PT decoder would indicate begin / end by a branch
from / to zero. That hides useful information, in particular when a
trace ends with a call. Before remedying that, prepare 'perf script' to
display sample flags with more combinations that include trace begin /
end. In those cases display 'tr start' and 'tr end' separately.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20180920130048.31432-2-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>