Add a test for non-treelike behavior of stack (WebAssembly#961)

We've recently found a bug in a WebAssembly library we've been working
with where we're mapping WebAssembly to a tree-like IR internally. The
way we parse into this representation, however, has a bug when the
function isn't itself tree-like but rather exibits properties that
exploit a stack machine. For example this isn't so straightforward to
represent in a tree-like fashion:

    (import "" "a" (func $foo))
    (import "" "b" (func $foo (result i32)))
    (func (result i32)
      call $b
      call $b
      call $a
      i32.xor)

The extra `call $a` in the middle is valid `WebAssembly` but needs
special treatment when converting to a more tree-like IR format. I
figured it'd be good to ensure there's a spec test covering this case as
we currently pass the suite of spec tests but still contain this bug!

Author: alexcrichton

test/core/stack.wast

@@ -125,12 +125,31 @@
     end
     local.get $res
   )
+
+  (global $temp (mut i32) (i32.const 0))
+  (func $add_one_to_global (result i32)
+    (local i32)
+    (global.set $temp (i32.add (i32.const 1) (global.get $temp)))
+    (global.get $temp)
+  )
+  (func $add_one_to_global_and_drop
+    (drop (call $add_one_to_global))
+  )
+  (func (export "not-quite-a-tree") (result i32)
+    call $add_one_to_global
+    call $add_one_to_global
+    call $add_one_to_global_and_drop
+    i32.add
+  )
 )
 
 (assert_return (invoke "fac-expr" (i64.const 25)) (i64.const 7034535277573963776))
 (assert_return (invoke "fac-stack" (i64.const 25)) (i64.const 7034535277573963776))
 (assert_return (invoke "fac-mixed" (i64.const 25)) (i64.const 7034535277573963776))
 
+(assert_return (invoke "not-quite-a-tree") (i32.const 3))
+(assert_return (invoke "not-quite-a-tree") (i32.const 9))
+
 
 ;; Syntax of flat call_indirect