[Wasm GC] Fix MergeBlocks on BrOn (#3702)

The pass was only aware of Break and Switch. Refactor it to use the
generic code, so that we can first handle Break, and then if anything
remains, note a problem was found. The same path can handle a Switch
which we handled before and also a BrOn etc.

git diff is not that useful after the refactoring sadly, but basically this just
moves the Break code and the Drop code, then adds the BranchUtils::operateOn
stuff after them (and we switch to a unified visitor so that we get called
for all expressions).

0 files changed, 0 insertions, 0 deletions