Stop explicitly scheduling domfrontier before the loop passes,

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112420 91177308-0d34-0410-b5e6-96231b3b80d8