[Reduce] Rewrite runDeltaPass() workloop: do reduce a single and/or last target

[Reduce] Rewrite runDeltaPass() workloop: do reduce a single and/or last target

Summary:
If there was a single target to begin with, because a single target
can only occupy a single chunk, we couldn't increase granularity.
and would immediately give up.

Likewise, if we had multiple targets, if by the end we'd end up with
a single target, we wouldn't finish reducing it, it would always
end up being "interesting"

Reviewers: dblaikie, nickdesaulniers, diegotf

Reviewed By: dblaikie

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84318