Have dead-branch-elim handle conditional exits from selections. (#1850)

When dead-branch-elim folds a conditional branch, it also deletes the
OpSelectionMerge instruction.  If that construct contains a
conditional branch to the merge node, it will not have its own
OpSelectionMerge.  When the headers merge instruction is deleted, the
the inner conditional branch will no longer be legal.  It will be a
selection to a node that is not a merge node.

We fix this up by moving the OpSelectionMerge to a new location if it is
still needed.