[Branch Hints] Add a utility to compare with metadata, and use it in merging opt passes

[kripken]

Several passes merge code, and it is dangerous to merge identical code
with different branch hints (or other code annotations): the different
parts of code may intentionally have different hints, e.g. an if that checks
a condition and then runs the same code, knowing that if the condition
was true the hints go one way, and vice versa.

[tlively]

It might be better to merge and drop the hint in this case. If the only benefit of branch hints is that cold code can be placed far away, then surely just deduplicating the cold code into some warmer code is at least as good.

[kripken]

But the issue may be cold code inside each of the arms. Imagine that under some condition, matching$x, the first arm runs 30% faster (because the internal if is almost never entered, depending on $y), and that in the reverse condition, the second arm runs 30% faster (because the internal if is almost always entered). Merging the arms and removing the branch hint would make us 30% slower.