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).

1 files changed, 3 insertions, 4 deletions

diff --git a/test/passes/remove-unused-brs_enable-multivalue.txt b/test/passes/remove-unused-brs_enable-multivalue.txt
index 9df644daa..0a66153e9 100644
--- a/test/passes/remove-unused-brs_enable-multivalue.txt
+++ b/test/passes/remove-unused-brs_enable-multivalue.txt
@@ -2502,8 +2502,9 @@
   (i32.const 0)
  )
  (func $ifs-copies-recursive (param $20 i32) (result i32)
-  (local.set $20
-   (select
+  (if
+   (i32.const 1)
+   (local.set $20
     (select
      (select
       (i32.const 4)
@@ -2513,8 +2514,6 @@
      (local.get $20)
      (i32.const 2)
     )
-    (local.get $20)
-    (i32.const 1)
    )
   )
   (local.get $20)