getType() called on DISCARD node

From http://mail.openjdk.java.net/pipermail/nashorn-dev/2013-October/002291.html:

The left-hand side of a BinaryNode does not have a Type if it is a Discard node, that means `lhs.getType()` may throw an assertion error. Moving the Type.widest() calls makes sure `lhs.getType()` is only called when it is safe to do.

test case:
/* no assertion */ function f() { if(x3, y) x; }