[LoopSink] Make the enforcement of determinism deterministic.

LoopBlockNumber is a DenseMap<BasicBlock*, int>, comparing the result of
find() will compare a pair<BasicBlock*, int>. That's of course depending
on pointer ordering which varies from run to run. Reverse iteration
doesn't find this because we're copying to a vector first.

This bug has been there since 2016 but only recently showed up on clang
selfhost with FDO and ThinLTO, which is also why I didn't manage to get
a reasonable test case for this. Add an assert that would've caught
this.

llvm-svn: 336439