[Float2Int] Make iteration over Roots deterministic

Summary:
Use a SmallSetVector instead of a SmallPtrSet when collecting
and storing Roots.

The iteration order for a SmallPtrSet is not deterministic,
so in the past the order of items inserted in the WorkList
inside walkBackwards has been non-deterministic. This patch
intends to make the order of rewrites done in Float2Int
deterministic by changing the container for the Roots set.

The semantics result of the transformation should not be
any different afaict. But at least naming of IR variables
(when outputting the result as an ll file) should be more
stable now.

Reviewers: craig.topper, spatel, cameron.mcinally

Reviewed By: spatel

Subscribers: mgrang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74534