remove duplicate returns in end of function

[airballonboy]

if you returned at the end of a function you used to get an extra return 0 in gas_x86_64
for example the code

main () {
    return(0);
}

would generate

main:
    movq $0, %rax
    movq %rbp, %rsp
    popq %rbp
    ret
    movq $0, %rax
    movq %rbp, %rsp
    popq %rbp
    ret

to avoid a function exiting without a return but i made it check for if the function has returned in the last op and if not it will generate the code

[rexim]

I feel like this kinda of stuff should be the responsibility of the dead-code elimination when we implement it.

[airballonboy]

I feel like this kinda of stuff should be the responsibility of the dead-code elimination when we implement it.

Yeah makes sense, But maybe we should have this until the full feature is implemented.