[llvm] Iterate SmallPtrSet in reverse order to uncover non-determinism in codegen

Summary:
Given a flag (-mllvm -reverse-iterate) this patch will enable iteration of SmallPtrSet in reverse order.
The idea is to compile the same source with and without this flag and expect the code to not change.
If there is a difference in codegen then it would mean that the codegen is sensitive to the iteration order of SmallPtrSet.
This is enabled only with LLVM_ENABLE_ABI_BREAKING_CHECKS.

Reviewers: chandlerc, dexonsmith, mehdi_amini

Subscribers: mgorny, emaste, llvm-commits

Differential Revision: https://reviews.llvm.org/D26718

llvm-svn: 289619