[llvm-exegesis] Dry run mode

[llvm-exegesis] Dry run mode

Sometimes we only want to ensure that we can produce snippets (all the way
through `SnippetRepetitor`!), but don't care for the execution.
E.g. all of our tests are this way.

I've built LLVM without PFM and removed my CPU from `X86PfmCounters.td`,
and this produces the expected results in that configuration.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D139448