fix two strange things in the default passmgr:

1. we'd run simplifycfg at the very start, even though
the per function passes have already cleaned this up.

2. In the main per-function pipeline that is interlaced with inlining 
   etc, we would do instcombine, jump threading, simplifycfg *before*
   doing SROA.  SROA is much more likely to expose opportunities for
   these passes than they are for SROA, so move SRoA up earlier.

also add some comments.

llvm-svn: 85742