compiler: avoid copy for string([]byte) conversion used in map keys

compiler: avoid copy for string([]byte) conversion used in map keys

    If a string([]byte) conversion is used immediately as a key for a
    map read, we don't need to copy the backing store of the byte
    slice, as mapaccess does not keep a reference to it.

    The gc compiler does more than this: it also avoids the copy if
    the map key is a composite literal that contains the conversion
    as a field, like, T{ ... { ..., string(b), ... }, ... }. For now,
    we just optimize the simple case, which is probably most common.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/176197

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

From-SVN: r271044