Change the order branches are simplified in dead branch elim (#2728)

Dead branch elimination needs to know about the constructs that a block is contained it when determining what to do with its merge instruction.  We currently fold branches in block as we see them, which is parent constructs before their children.  This causes the struct cfg analysis to crash because it tries to get the parent construct for a block after the parent has been folded.

This can be fixed by folding the branch of the children before the parents.

Fixes #2667.