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/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt b/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt
index 48a5b48f5..ada155fd6 100644
--- a/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt
+++ b/test/passes/generate-stack-ir_optimize-stack-ir_print-stack-ir_all-features.txt
@@ -26,8 +26,10 @@
  (func $eh (; has Stack IR ;)
   (local $exn exnref)
   (try
-   (throw $e0
-    (i32.const 0)
+   (do
+    (throw $e0
+     (i32.const 0)
+    )
    )
    (catch
     (local.set $exn