Support br_on_cast null (#5397)

As well as br_on_cast_fail null. Unlike the existing br_on_cast* instructions,
these new instructions treat the cast as succeeding when the input is a null.
Update the internal representation of the cast type in `BrOn` expressions to be
a `Type` rather than a `HeapType` so it will include nullability information.

Also update and improve `RemoveUnusedBrs` to handle the new instructions
correctly and optimize in more cases.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index 81e3ae989..c1064b7db 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -2143,13 +2143,19 @@ struct PrintExpressionContents
         printMedium(o, "br_on_cast ");
         printName(curr->name, o);
         o << ' ';
-        printHeapType(o, curr->intendedType, wasm);
+        if (curr->castType.isNullable()) {
+          printMedium(o, "null ");
+        }
+        printHeapType(o, curr->castType.getHeapType(), wasm);
         return;
       case BrOnCastFail:
         printMedium(o, "br_on_cast_fail ");
         printName(curr->name, o);
         o << ' ';
-        printHeapType(o, curr->intendedType, wasm);
+        if (curr->castType.isNullable()) {
+          printMedium(o, "null ");
+        }
+        printHeapType(o, curr->castType.getHeapType(), wasm);
         return;
       case BrOnFunc:
         printMedium(o, "br_on_func ");