Merge pull request #467 from WebAssembly/add_nop

Add nop

Author: titzer

AstSemantics.md

@@ -231,6 +231,7 @@ WebAssembly offers basic structured control flow with the following constructs.
 Since all AST nodes are expressions in WebAssembly, control constructs may yield
 a value and may appear as children of other expressions.
 
+ * `nop`: an empty operator that does not yield a value 
  * `block`: a fixed-length sequence of expressions with a label at the end
  * `loop`: a block with an additional label at the beginning which may be used to form loops
  * `if`: if expression with a *then* expression
@@ -255,7 +256,7 @@ loop from outside it. This restriction ensures all control flow graphs are well-
 
 ### Yielding values from control constructs
 
-The `if`, `br`, `br_if`, `case`, and `return` constructs do not yield values.
+The `nop`, `if`, `br`, `br_if`, `case`, and `return` constructs do not yield values.
 Other control constructs may yield values if their subexpressions yield values:
 
 * `block`: yields either the value of the last expression in the block or the result of an inner `br` that targeted the label of the block