[WebAssembly] Fix fixEndsAtEndOfFunction for try-catch

When the function return type is non-void and `end` instructions are at
the very end of a function, CFGStackify's `fixEndsAtEndOfFunction`
function fixes the corresponding block/loop/try's type to match the
function's return type. This is applied to consecutive `end` markers at
the end of a function. For example, when the function return type is
`i32`,
```
block i32    ;; return type is fixed to i32
  ...
  loop i32   ;; return type is fixed to i32
    ...
  end_loop
end_block
end_function
```

But try-catch is a little different, because it consists of two parts:
a try part and a catch part, and both parts' return type should satisfy
the function's return type. Which means,
```
try i32      ;; return type is fixed to i32
  ...
  block i32  ;; this should be changed i32 too!
    ...
  end_block
catch
  ...
end_try
end_function
```
As you can see in this example, it is not sufficient to only `end`
instructions at the end of a function; in case of `try`, we should
check instructions before `catch`es, in case their corresponding `try`'s
type has been fixed.

This changes `fixEndsAtEndOfFunction`'s algorithm to use a worklist
that contains a reverse iterator, each of which is a starting point for
a new backward `end` instruction search.

Fixes https://bugs.llvm.org/show_bug.cgi?id=47413.

Reviewed By: dschuff, tlively

Differential Revision: https://reviews.llvm.org/D87207

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