[CallSiteSplitting] Report edge deletion to DomTreeUpdater

Summary:
When splitting musttail calls, the split blocks' original terminators
get removed; inform the DTU when this happens.

Also add a testcase that fails an assertion in the DTU without this fix.


Reviewers: fhahn, junbuml

Reviewed By: fhahn

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D55027

llvm-svn: 347872