Make try body start with 'do' (#2846)

In WebAssembly/exception-handling#52, We decided to put `try` bodies in
a `do` clause to be more consistent with `catch`.

- Before
```wast
(try
  ...
  (catch
    ...
  )
)
```

- After
```wast
(try
  (do
    ...
  )
  (catch
    ...
  )
)
```

Another upside of this change is when there are multiple instructions
within a `try` body, we no longer need to wrap them in a `block`.

1 files changed, 4 insertions, 2 deletions

diff --git a/test/passes/optimize-instructions_all-features.wast b/test/passes/optimize-instructions_all-features.wast
index 5078e74c5..9694d9b28 100644
--- a/test/passes/optimize-instructions_all-features.wast
+++ b/test/passes/optimize-instructions_all-features.wast
@@ -240,9 +240,11 @@
     )
     (if
       (try (result i32)
-        (i32.eqz
+        (do
           (i32.eqz
-            (i32.const 123)
+            (i32.eqz
+              (i32.const 123)
+            )
           )
         )
         (catch