Summary:
If we had 2 threads on the run queue, say [A,B], and B is bound to the
current Task, then we would fail to migrate any threads.  This fixes it
so that we would migrate A in that case.

This will help parallelism a bit in programs that have lots of bound
threads.

Test Plan:
Test program in #12419, which is actually not a great program but it
does behave a bit better after this change.

Reviewers: ezyang, niteria, bgamari, austin, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2430

GHC Trac Issues: #12419