Teach machine sink to

Teach machine sink to
1) Do forward copy propagation. This makes it easier to estimate the cost of the
   instruction being sunk.
2) Break critical edges on demand, including cases where the value is used by
   PHI nodes.
Critical edge splitting is not yet enabled by default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114227 91177308-0d34-0410-b5e6-96231b3b80d8