[CodeGenPrepare] Fix for using outdated/corrupt LoopInfo

Some transformation in CodeGenPrepare pass may create and/or delete
basic block, but they don't update the LoopInfo, so the LoopInfo may
end up containing dangling pointers and sometimes reused basic blocks,
which leads to "interesting" non-deterministic behaviour.

These transformations do not seem to alter the loop structure of the
function, and updating the loop info is quite straighforward.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D150384

Change-Id: If8ab3905749ea6be94fbbacd54c5cfab5bc1fba1