Add an option to disable codegen prepare critical edge splitting. In theory, PHI...

Add an option to disable codegen prepare critical edge splitting. In theory, PHI elimination is already doing all (most?) of the splitting needed. But machine-licm and machine-sink seem to miss some important optimizations when splitting is disabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111224 91177308-0d34-0410-b5e6-96231b3b80d8