[StructurizeCfg] Update dominator info.

In some cases StructurizeCfg updates root node, but dominator info
remains unchanges, it causes crash when expensive checks are enabled.
To cope with this problem a new method was added to DominatorTreeBase
that allows adding new root nodes, it is called in StructurizeCfg to
put dominator tree in sync.

This change fixes PR27488.

Differential Revision: https://reviews.llvm.org/D28114

llvm-svn: 291530