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, 13 insertions, 7 deletions

diff --git a/test/passes/remove-unused-names_optimize-instructions_all-features.txt b/test/passes/remove-unused-names_optimize-instructions_all-features.txt
index 84405cd17..182916755 100644
--- a/test/passes/remove-unused-names_optimize-instructions_all-features.txt
+++ b/test/passes/remove-unused-names_optimize-instructions_all-features.txt
@@ -12,7 +12,9 @@
   (local $x3 i32)
   (local.set $x0
    (try (result i32)
-    (i32.const 1)
+    (do
+     (i32.const 1)
+    )
     (catch
      (drop
       (exnref.pop)
@@ -26,7 +28,7 @@
   )
   (local.set $x1
    (try (result i32)
-    (block (result i32)
+    (do
      (call $dummy)
      (i32.const 1)
     )
@@ -46,10 +48,12 @@
   )
   (local.set $x2
    (try (result i32)
-    (block (result i32)
+    (do
      (try
-      (throw $e
-       (i32.const 0)
+      (do
+       (throw $e
+        (i32.const 0)
+       )
       )
       (catch
        (drop
@@ -72,9 +76,11 @@
   )
   (local.set $x3
    (try (result i32)
-    (block (result i32)
+    (do
      (try
-      (nop)
+      (do
+       (nop)
+      )
       (catch
        (drop
         (exnref.pop)