Rewrite a test to check actual output rather than intermediate implementation

Rewrite a test to check actual output rather than intermediate implementation
detail.

The was this test was written, it was relying on an implementation detail
(fixups) and hence was very brittle (relying, among other things, on the
exact ordering of statistics printed by MC).

The test was rewritten to check a more observable output difference. While it
doesn't cover 100% of the things the original test covered, it's a good
practice to write regression tests this way. If we want to check that
internal details and invariants hold, such tests should be expressed as unit
tests.

llvm-svn: 176377