[llvm-exegesis] Fix LBR checks/test
This patch fixes the LBR check in the local lit config. The test would
segfault as the loop body would be basically empty, causing a divide by
zero error. More investigation is needed there so we do not actually hit
that assertion and report a cleaner error somewhere. Specifying an
actual opcode to benchmark fixes the problem. The test would also fail
as -mcpu was set to the default x86 CPU rather than the one currently
being run on, so it would always fail to find a perf counter. This patch
fixes that by simply removing the -mcpu flag.
Given these issues, I'm not sure these tests have ever run in the ~5
years they have been in tree. There were some issues reported in
\#132861, so I guess we'll see if there are further issues when the
testing becomes more broad.
