Stop explicitly scheduling domfrontier before the loop passes,

since none of them use it.  With this, we now only run 
domfrontier (an N^2 analysis) 3 times at clang -O3: once for 
"early" per-function cleanup, once at the start of the
per-function pipeline to support SRoA, and once late because
the EHPrepare class uses it.

EHPrepare needs to stop using it, this is silly and wasteful.

llvm-svn: 112420