Add basic exception handling support (#2282)

This adds basic support for exception handling instructions, according
to the spec:
https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md

This PR includes support for:
- Binary reading/writing
- Wast reading/writing
- Stack IR
- Validation
- binaryen.js + C API
- Few IR routines: branch-utils, type-updating, etc
- Few passes: just enough to make `wasm-opt -O` pass
- Tests

This PR does not include support for many optimization passes, fuzzer,
or interpreter. They will be follow-up PRs.

Try-catch construct is modeled in Binaryen IR in a similar manner to
that of if-else: each of try body and catch body will contain a block,
which can be omitted if there is only a single instruction. This block
will not be emitted in wast or binary, as in if-else. As in if-else,
`class Try` contains two expressions each for try body and catch body,
and `catch` is not modeled as an instruction. `exnref` value pushed by
`catch` is get by `pop` instruction.

`br_on_exn` is special: it returns different types of values when taken
and not taken. We make `exnref`, the type `br_on_exn` pushes if not
taken, as `br_on_exn`'s type.

1 files changed, 37 insertions, 1 deletions

diff --git a/test/exception-handling.wast.fromBinary.noDebugInfo b/test/exception-handling.wast.fromBinary.noDebugInfo
index 11555cd10..23ba7946c 100644
--- a/test/exception-handling.wast.fromBinary.noDebugInfo
+++ b/test/exception-handling.wast.fromBinary.noDebugInfo
@@ -1,8 +1,44 @@
 (module
  (type $0 (func (param exnref) (result exnref)))
- (memory $0 1 1)
+ (type $1 (func))
+ (type $2 (func (param i32)))
+ (event $event$0 (attr 0) (param i32))
  (func $0 (; 0 ;) (type $0) (param $0 exnref) (result exnref)
   (local.get $0)
  )
+ (func $1 (; 1 ;) (type $1)
+  (local $0 exnref)
+  (try
+   (throw $event$0
+    (i32.const 0)
+   )
+   (catch
+    (local.set $0
+     (exnref.pop)
+    )
+    (drop
+     (block $label$3 (result i32)
+      (rethrow
+       (br_on_exn $label$3 $event$0
+        (local.get $0)
+       )
+      )
+     )
+    )
+   )
+  )
+  (block $label$4
+   (try
+    (br $label$4)
+    (catch
+     (drop
+      (exnref.pop)
+     )
+     (br $label$4)
+    )
+   )
+  )
+  (unreachable)
+ )
 )