If a string([]byte) conversion is used immediately in a string
    concatenation, we don't need to copy the backing store of the
    byte slice, as the runtime function doesn't hold any reference
    to it.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/182437

	* go.dg/concatstring.go: New test.

From-SVN: r272460