Fix binop related crashes in the interpreter

Fix binop related crashes in the interpreter

if (foo instanceof blah) would crash because we generated a BinOp instruction
with a null aluop pointer. Instanceof, in and add are binops that now require
a context, and thus require a different instruction (BinOpContext). This was
already handled by the traditional binop() of the isel, but not by the binop
expression that can be in a cjump. Centralize the code by calling a common
binop helper from isel binop as well as cjump.

Change-Id: I793ee3eebe56db4c86d5399a783a84be3093fd35
Reviewed-by: Lars Knoll <lars.knoll@digia.com>