[PM] Use range-based for loops in LegacyPassManager.cpp (NFC).

Summary:
This patch replaces a bunch of iterator-based for loops with range-based
for loops. There are 2 iterator-based loops left in this file in
removeNotPreservedAnalysis, but I think those cannot be replaced by
range-based for loops as they modify the container they are iterating
over.

Unless I missed something, this schould be a NFC and I would appreciate
if someone could have a quick look to confirm that.

Reviewers: chandlerc, pcc, jhenderson

Reviewed By: jhenderson

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D35310

llvm-svn: 307902