[LoopDeletion] NFC: Move phi node value setting into prepass

Recommit NFC patch (rL306157) where I missed incrementing the basic block iterator,
which caused loop deletion tests to hang due to infinite loop.
Had reverted it in rL306162.

rL306157 commit message:
Currently, the implementation of delete dead loops has a special case
when the loop being deleted is never executed. This special case
(updating of exit block's incoming values for phis) can be
run as a prepass for non-executable loops before performing
the actual deletion.

llvm-svn: 306254