Introduce a loop block rotation optimization to the new block placement

Introduce a loop block rotation optimization to the new block placement
pass. This is designed to achieve one of the important optimizations
that the old code placement pass did, but more simply.

This is a somewhat rough and *very* conservative version of the
transform. We could get a lot fancier here if there are profitable cases
to do so. In particular, this only looks for a single pattern, it
insists that the loop backedge being rotated away is the last backedge
in the chain, and it doesn't provide any means of doing better in-loop
placement due to the rotation. However, it appears that it will handle
the important loops I am finding in the LLVM test suite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145158 91177308-0d34-0410-b5e6-96231b3b80d8