compiler: stack allocate a buffer for non-escaping string ops

compiler: stack allocate a buffer for non-escaping string ops

    For string concatenation, string to/from byte or rune slice
    conversion, and int to string conversion, if the result does not
    escape, we can allocate a small (32-element, or 4-byte for int to
    string) buffer on stack, and pass it to the runtime function. If
    the result fits in the buffer, it doesn't need to do a heap
    allocation.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/182538

From-SVN: r272468