[llvm][codegen] Fix non-determinism in StackFrameLayoutAnalysisPass output

We were iterating over a SmallPtrSet when outputting slot variables.
This is still correct but made the test fail under reverse iteration.
This patch replaces the SmallPtrSet with a SmallVector.

Also remove the "Stack Frame Layout" lines from arm64-opt-remarks-lazy-bfi test,
since those also break under reverse iteration.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D142127