StructurizeCFG: Fix inverting a branch on an argument

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195492 91177308-0d34-0410-b5e6-96231b3b80d8
StructurizeCFG: Fix verification failure with some loops.

If the beginning of the loop was also the entry block
of the function, branches were inserted to the entry block
which isn't allowed. If this occurs, create a new dummy
function entry block that branches to the start of the loop.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195493 91177308-0d34-0410-b5e6-96231b3b80d8