[GlobalOpt] Preserve CFG analyses

The only place we modify the CFG is when calling
removeUnreachableBlocks(), so insert a callback there which invalidates
analyses for that function (or recomputes DT in the legacy PM).

Small compile time wins across the board:
https://llvm-compile-time-tracker.com/compare.php?from=f444ea8ce0aaaa5ec1a4129809389da15cc41396&to=698f41f4fc26cbf1006ed5d88e9d658edfc5b749&stat=instructions

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D128145