Fix dead branch elim infinite loop. (#1997)

When looking for a break from a selection construct, we do not need to
look inside nested constructs.  However, if a loop header has an
unconditional branch, then we enter the loop.  Entering the loop causes
an infinite loop because we keep going through the loop.

The solution is to look for a merge block, if one exsits, even for block
terminated by an OpBranch.

Fixes #1979.