MergeBlocks: Allow side effects in a ternary's first element (#4238)

Side effects in the first element are always ok there, as they are
not moved across anything else: they happen before their parent
both before and after the opt.

The pass just left ternary as a TODO, so do at least one part of
that now (we can do the rest as well, with some care).

This is fairly useful on array.set which has 3 operands, and the
first often has interesting things in it.

1 files changed, 2 insertions, 6 deletions

diff --git a/src/passes/MergeBlocks.cpp b/src/passes/MergeBlocks.cpp
index 9d797ae0a..b77dbef83 100644
--- a/src/passes/MergeBlocks.cpp
+++ b/src/passes/MergeBlocks.cpp
@@ -524,14 +524,10 @@ struct MergeBlocks
                        Expression*& first,
                        Expression*& second,
                        Expression*& third) {
-    // TODO: for now, just stop when we see any side effect. instead, we could
-    //       check effects carefully for reordering
     Block* outer = nullptr;
-    if (EffectAnalyzer(getPassOptions(), *getModule(), first)
-          .hasSideEffects()) {
-      return;
-    }
     outer = optimize(curr, first, outer);
+    // TODO: for now, just stop when we see any side effect after the first
+    //       item, but we could handle them carefully like we do for binaries.
     if (EffectAnalyzer(getPassOptions(), *getModule(), second)
           .hasSideEffects()) {
       return;