Move non-constant folding parts of fold_constants into a separate functions.

Move non-constant folding parts of fold_constants into a separate functions.

The non-constant folding parts of fold_constants are moved into
separate functions. op_integerize handles converting ops to integer
(and special case of OP_NEGATE), op_std_init handling some standard
functionality (forced scalar context and allocating the TARGET).
Both functions are called where fold_constants is called (but we might
want to make that a bit some selective and use op_std_init in other
places).