Codegen:Do not invalidate dominator tree when bailing out during code generation

When codegenerating invariant loads in some rare cases we cannot generate code
and bail out. This change ensures that we maintain a valid dominator tree
in these situations. This fixes llvm.org/PR26736

Contributed-by: Matthias Reisinger <d412vv1n@gmail.com>
llvm-svn: 264142