[Codegen] Disable Polly's codegen verification by default

As has been reported in the previous commit, codegen verification can result in
quadratic compile time increases for large functions with many scops. This is
certainly not something we would like to have in the Polly default
configuration. Hence, we disable codegen verification by default -- also to see
if this resolves some of the compilation timeouts we currently see on the AOSP
buildbots. We still leave this feature in Polly as it has shown _very_ useful
for debugging. In fact, we may want to have a discussion if we can bring this
feature back in a way that does not impact compilation time so much.

Thanks to Eli Friedman <efriedma@codeaurora.org> for reporting this issue and
for providing the test case in the previous commit (where I forgot to
acknowledge him).

llvm-svn: 301670