perf script: Fix off by one in brstackinsn IPC computation

When we hit the end of a program block, need to count the last
instruction too for the IPC computation. This caused large errors for
small blocks.

  % perf script -b ls / > /dev/null

Before:

  % perf script -F +brstackinsn --xed
  ...
        00007f94c9ac70d8                        jz 0x7f94c9ac70e3                       # PRED 3 cycles [36] 4.33 IPC
        00007f94c9ac70e3                        testb  $0x20, 0x31d(%rbx)
        00007f94c9ac70ea                        jnz 0x7f94c9ac70b0
        00007f94c9ac70ec                        testb  $0x8, 0x205ad(%rip)
        00007f94c9ac70f3                        jz 0x7f94c9ac6ff0               # PRED 1 cycles [37] 3.00 IPC

After:

  % perf script -F +brstackinsn --xed
  ...
        00007f94c9ac70d8                        jz 0x7f94c9ac70e3                       # PRED 3 cycles [15] 4.67 IPC
        00007f94c9ac70e3                        testb  $0x20, 0x31d(%rbx)
        00007f94c9ac70ea                        jnz 0x7f94c9ac70b0
        00007f94c9ac70ec                        testb  $0x8, 0x205ad(%rip)
        00007f94c9ac70f3                        jz 0x7f94c9ac6ff0               # PRED 1 cycles [16] 4.00 IPC

Suggested-by: Denis Bakhvalov <denis.bakhvalov@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20190711181922.18765-2-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>