[DomTree] Assert that blocks in queries aren't from another function

This assertion should help us catch cases when DT is used in a way that
doesn't make much sense and usually indicates usage errors. In D110752
you can see a test on which this assertion catches a miscompile.

The assertion is added to getNode since all queries seem to be
routed through that function for all non-trivial cases.

Reviewed By: aeubanks, MaskRay
Differential Revision: https://reviews.llvm.org/D110751