MergeBlocks: Rewrite to use a generic algorithm (#4323)

Before this we had special logic for various call types. This replaces all
that with a single general code path, which unifies everything except
for control flow constructs (which remain as before, handled in a
special way for each of them).

The algorithm is simple and direct, basically it goes through the
children and when it finds a block, it sees if it can move the block's
contents outside of the parent. While doing so it takes into account
effects and so forth.

To make this easy, a random-access API is added to ChildIterator.

Diff without whitespace makes the existing test updates a lot simpler.
Note that this is not NFC as the old algorithm had some quirks like
not taking into account effects when there were more than 2 children;
the new code is uniform in how it handles things.

This ends up removing 19% of all blocks in j2wasm, which reduces
1% of total code size.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/passes/ReorderFunctions.cpp b/src/passes/ReorderFunctions.cpp
index 66b8275ef..326893951 100644
--- a/src/passes/ReorderFunctions.cpp
+++ b/src/passes/ReorderFunctions.cpp
@@ -24,7 +24,7 @@
 // a less beneficial position for compression, that is, mutually-compressible
 // functions are no longer together (when they were before, in the original
 // order, the has some natural tendency one way or the other). TODO: investigate
-// similarity ordering here.
+// similarity ordering here (see #4322)
 //
 
 #include <memory>