[CodeGen] Introduce MachineBasicBlock::replacePhiUsesWith helper and use it. NFC

Summary:
Found a couple of places in the code where all the PHI nodes
of a MBB is updated, replacing references to one MBB by
reference to another MBB instead.

This patch simply refactors the code to use a common helper
(MachineBasicBlock::replacePhiUsesWith) for such PHI node
updates.

Reviewers: t.p.northover, arsenm, uabelho

Subscribers: wdng, hiraditya, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66750

llvm-svn: 370463