Implement full support for promoting allocas to registers using SSAUpdater

Implement full support for promoting allocas to registers using SSAUpdater
instead of DomTree/DomFrontier.  This may be interesting for reducing compile
time.  This is currently disabled, but seems to work just fine.

When this is enabled, we eliminate two runs of dominator frontier, one in the
"early per-function" optimizations and one in the "interlaced with inliner"
function passes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123434 91177308-0d34-0410-b5e6-96231b3b80d8