cmd/compile: don't generate algs for ... args

Note that this is only safe because
the compiler generates multiple distinct
gc.Types. If we switch to having canonical
gc.Types, then this will need to be updated
to handle the case in which the user uses both
map[[n]T]S and also calls a function f(...T) with n arguments.
In that case, the runtime needs algs for [n]T, but this could
mark the sole [n]T type as Noalg. This is a general
problem with having a single bool to represent
whether alg generation is needed for a type.

Cuts 17k off cmd/go and 13k off golang.org/x/tools/cmd/godoc,
approx 0.14% and 0.07% respectively.

For #6853 and #9930

Change-Id: Iccb6b9fd88ade5497d7090528a903816d340bf0a
Reviewed-on: https://go-review.googlesource.com/19770Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/walk.go

@@ -1739,6 +1739,7 @@ func mkdotargslice(lr0, nn []*Node, l *Field, fp int, init *Nodes, ddd *Node) []
 	}
 
 	tslice := typSlice(l.Type.Elem())
+	tslice.Noalg = true
 
 	var n *Node
 	if len(lr0) == 0 {