FCAs. This is essential in order to promote allocas that are used in
struct returns by frontends like Clang. The FCA load would block the
rest of the pass from firing, resulting is significant regressions with
the bullet benchmark in the nightly test suite.

Thanks to Duncan for repeated discussions about how best to do this, and
to both him and Benjamin for review.

This appears to have blocked many places where the pass tries to fire,
and so I'm expect somewhat different results with this fix added.

As with the last big patch, I'm including a change to enable the SROA by
default *temporarily*. Ben is going to remove this as soon as the LNT
bots pick up the patch. I'm just trying to get a round of LNT numbers
from the stable machines in the lab.

NOTE: Four clang tests are expected to fail in the brief window where
this is enabled. Sorry for the noise!

llvm-svn: 164119