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
