V4 runtime: tune Runtime::add/sub/mul a bit.

Tell the compiler to schedule the int32 case first, tune the double
conversion a bit (int64->double is quite expensive), and write the
function in such a way that it matches typical overflow idiom which
compilers recognize.

Change-Id: Ieae9a60275716002fbdbc54e1d7291c8aad8c927
Reviewed-by: Lars Knoll <lars.knoll@digia.com>