compiler: stack allocate storage for temp slices.

compiler: stack allocate storage for temp slices.

    During the lowering phase, the variable arguments to a varargs call are
    packaged up into a temporary slice object; the storage for this slice
    was being unconditionally allocated on the heap.  Heap allocation is not
    necessary, however, if the varargs call correspond to an "append", since
    the append runtime routine only reads the slice storage (as opposed to
    stashing away the storage pointer). Enhance the lowering code to keep
    the slice storage on the stack for append() calls, to improve
    performance.

    Addresses issue golang/go#17304.

    Reviewed-on: https://go-review.googlesource.com/30136

From-SVN: r240853