The builtin copy function is lowered to runtime functions
    slicecopy, stringslicecopy, or typedslicecopy. The first two are
    basically thin wrappers of memmove. Instead of making a runtime
    call, we can just use __builtin_memmove. This gives the compiler
    backend opportunities for further optimizations.
    
    Move the lowering of builtin copy function to flatten phase for
    the ease of rewriting.
    
    Also do this optimization for the copy part of append(s1, s2...).
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/170005

From-SVN: r271017