Recommit "[LoopUnroll] Directly update DT instead of DTU."

This reverts commit c5ea42bc.

Recommit the patch with a fix for loops where the exiting terminator is
not a branch instruction. In that case, ExitInfos may be empty. In
addition to checking if there's a single exiting block also check if
there's a single ExitInfo.

A test case has been added in f92b3539.