StructurizeCFG: simplify phi nodes when possible

After structurization, some phi nodes can have a single incoming edge
and can be simplified away. This change runs a simplify query on all
phis that are either modified or added by the structurizer. This also
moves some phis closer to their use as a side benefit.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D75500