py/compile: Add COMP_RETURN_IF_EXPR option to enable return-if-else opt.

With this optimisation enabled the compiler optimises the if-else
expression within a return statement.  The optimisation reduces bytecode
size by 2 bytes for each use of such a return-if-else statement.  Since
such a statement is not often used, and costs bytes for the code, the
feature is disabled by default.

For example the following code:

    def f(x):
        return 1 if x else 2

compiles to this bytecode with the optimisation disabled (left column is
bytecode offset in bytes):

    00 LOAD_FAST 0
    01 POP_JUMP_IF_FALSE 8
    04 LOAD_CONST_SMALL_INT 1
    05 JUMP 9
    08 LOAD_CONST_SMALL_INT 2
    09 RETURN_VALUE

and to this bytecode with the optimisation enabled:

    00 LOAD_FAST 0
    01 POP_JUMP_IF_FALSE 6
    04 LOAD_CONST_SMALL_INT 1
    05 RETURN_VALUE
    06 LOAD_CONST_SMALL_INT 2
    07 RETURN_VALUE

So the JUMP to RETURN_VALUE is optimised and replaced by RETURN_VALUE,
saving 2 bytes and making the code a bit faster.