[Dominators] Faster EXPENSIVE_CHECKS builds

Hi folks,

In case you missed it and care about the speed of builds with EXPENSIVE_CHECKS enabled: they should get noticeably faster after r323298.

Before the patch, building the llvm test suite took 51m 42s, and after the patch it got down to 39m 7s (measured on 2x Intel E5-2690).

The patch introduced 3 DomTree verification levels (Fast, Basic, Full) to make the expensive checks more bearable – they now use the Basic verification level that doesn’t check the sibling property and compares the tree with a fresh one instead.
This is obviously faster, but can leave some construction bugs undetected in very rare cases. If you want to use the full DomTree verification with (or without) expensive checks, you can pass -mllvm -verify-dom-info to clang and get the previous behavior.

You can find more details in the code review here: https://reviews.llvm.org/D42337.

This could be also merged into the 6.0 branch, but I’m not sure if folks need it.