Currently, allocation expression that can be allocated on stack
    is implemented with __builtin_alloca, which turns into
    __morestack_allocate_stack_space, which may call C malloc. This
    may be slow. Also if this happens during certain runtime
    functions (e.g. write barrier), bad things might happen (when
    the escape analysis is enabled for the runtime). Make a temporary
    variable on stack for the allocation instead.
    
    Also remove the write barrier in the assignment in building heap
    expression when it is stack allocated.
    
    Reviewed-on: https://go-review.googlesource.com/86242

From-SVN: r256412