[NFC] BasicBlock: refactor changePhiUses() out of replacePhiUsesWith(), use it

Summary:
It is a common thing to loop over every `PHINode` in some `BasicBlock`
and change old `BasicBlock` incoming block to a new `BasicBlock` incoming block.
`replaceSuccessorsPhiUsesWith()` already had code to do that,
it just wasn't a function.
So outline it into a new function, and use it.

Reviewers: chandlerc, craig.topper, spatel, danielcdh

Reviewed By: craig.topper

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61013

llvm-svn: 359996