perf expr: Increase EXPR_MAX_OTHER to support metrics with more than 15 variables

perf expr: Increase EXPR_MAX_OTHER to support metrics with more than 15 variables

We have metrics that define more than 15 variables, like
Branch_Misprediction_Cost. Increasing the allowed variables count to 20.

As Andy pointed out, we can't go too high in here, because some of the
code has O(n^2) complexity (already_seen) and we might want to do some
other changes (like using hash tables) before increasing the maximum
even more.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Link: http://lore.kernel.org/lkml/20200228093616.67125-4-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>