[NFC] Simplify a bit of logic in RemoveUnusedBrs (#8603)

The checks in the old code were not needed: we have a block that
has a single child, another block. Any branch to the child sends a
value that flows out to the parent immediately. No circumstances
exist in which we can error.

(block $outer
  (block $inner
    ..code and a br to $inner, which can branch to $outer instead..
  )
)

Also, this code only handles blocks *without* a value, so even
subtyping is not an issue here. (Merging blocks with different
types is handled elsewhere, RemoveUnusedNames, so no
need to add new logic here.)

Author: kripken

src/passes/RemoveUnusedBrs.cpp

@@ -1224,11 +1224,7 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
           // if this block has just one child, a sub-block, then jumps to the
           // former are jumps to us, really
           if (auto* child = list[0]->dynCast<Block>()) {
-            // the two blocks must have the same type for us to update the
-            // branch, as otherwise one block may be unreachable and the other
-            // concrete, so one might lack a value
-            if (child->name.is() && child->name != curr->name &&
-                child->type == curr->type) {
+            if (child->name.is()) {
               redirectBranches(child, curr->name);
             }
           }