Avoid generating implicit jumps in if statements when they unconditionally return/jump

[Bananattack]

Currently, there is a small issue of suboptimal code generation when an if statement with an else clause contains a return statement in the if body. This appears to be either a regression, or an oversight, as this used to work in the legacy D versions of Wiz.

The if statement code generation should check if the last statement of the if statement body is an unconditional branch (goto, return, continue, break, etc. without a condition). If so, the if statement should omit any implicit branch at the end of the body:

In other words,

if carry {
    return;
} else {
  // do something
}

should become

    bcc @else
@if
    ret
@else
    ; do something
@end

rather than

    bcc @else
@if
    ret
    jmp @end
@else
    ; do something
@end

This will also fix the 6502_return_on_if_else_path.wiz test included in the block tests.