CodeGen: Assert that liveness is up to date when reading block live-ins.

Add an assert that checks whether liveins are up to date before they are
used.

- Do not print liveins into .mir files anymore in situations where they
  are out of date anyway.
- The assert in the RegisterScavenger is superseded by the new one in
  livein_begin().
- Skip parts of the liveness updating logic in IfConversion.cpp when
  liveness isn't tracked anymore (just enough to avoid hitting the new
  assert()).

Differential Revision: https://reviews.llvm.org/D27562

llvm-svn: 291169