cmd/compile/internal: slice/map literals janitoring

cmd/compile/internal: slice/map literals janitoring

Simplify slice/map literal expressions.
Caught with gofmt -d -s, fixed with gofmt -w -s
Checked that the result can still be compiled with Go 1.4.

Change-Id: I5c58801c20919618d2ad52b8e2380d53df2783f1
Reviewed-on: https://go-review.googlesource.com/13831
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>