Fix an ordering bug in the scalarizer.

I've added a new test case that causes the scalarizer to try and use
dead-and-erased values - caused by the basic blocks not being in
domination order within the function. To fix this, instead of iterating
through the blocks in function order, I walk them in reverse post order.

Differential Revision: https://reviews.llvm.org/D52540

llvm-svn: 344128