RemoveUnusedBrs: Allow less unconditional work and in particular division (#5989)

Fixes #5983: The testcase from there is used here in a new testcase
remove-unused-brs_levels in which we check if we are willing to unconditionally
do a division operation. Turning an if with an arm that does a division into a
select, which always does the division, is almost 5x slower, so we should probably
be extremely careful about doing that.

I took some measurements and have some suggestions for changes in this PR:

*   Raise the cost of div/rem to what I measure on my machine, which is 5x slower
    than an add, or worse.
*   For some reason we added the if arms rather than take the max of them, so
    fix that. This does not help the issue, but was confusing.
*   Adjust TooCostlyToRunUnconditionally in the pass from 9 to 8 (this helps
    balance the last point).
*   Use half that value when not optimizing for size. That is, we allow only 4 extra
    unconditional work normally, and 8 in -Os, and when -Oz then we allow any
    extra amount.

Aside from the new testcases, some existing ones changed. They all appear to
change in a reasonable way, to me.

We should perhaps go even further than this, and not even run a division
unconditionally in -Os, but I wasn't sure it makes sense to go that far as
other benchmarks may be affected. For now, this makes the benchmark in
#5983 run at full speed in -O3 or -Os, and it remains slow in -Oz. The
modified version of the benchmark that only divides in the if (no other
operations) is still fast in -O3, but it become slow in -Os as we do turn that
if into a select (but again, I didn't want to go that far as to overfit on that one
benchmark).

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