[MachineVerifier] retrofit iterators with range for. NFC

Summary:
Reviewing failures identified in D78586, I was finding the identifiers
for these iterators hard to read.

Reviewers: efriedma, MaskRay, jyknight

Reviewed By: MaskRay

Subscribers: hiraditya, llvm-commits, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78849