[IR] Add an iterator and range accessor for the PHI nodes of a basic
This allows writing much more natural and readable range based for loops
directly over the PHI nodes. It also takes advantage of the same tricks
for terminating the sequence as the hand coded versions.
I've replaced one example of this mostly to showcase the difference and
I've added a unit test to make sure the facilities really work the way
they're intended. I want to use this inside of SimpleLoopUnswitch but it
seems generally nice.
Differential Revision: https://reviews.llvm.org/D33533