cmd/compile: don't generate algs for map buckets

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[T]S and also map[[8]T]S. In that case,
the runtime needs algs for [8]T, but this could
mark the sole [8]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 5k off cmd/go and 22k off golang.org/x/tools/cmd/godoc,
approx 0.04% and 0.12% respectively.

For golang#6853 and golang#9930

Change-Id: I30a15ec72ecb62e2aa053260a7f0f75015fc0ade

Author: josharian

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

@@ -87,6 +87,7 @@ func mapbucket(t *Type) *Type {
 	arr.Type = keytype
 	arr.Bound = BUCKETSIZE
 	field = append(field, makefield("keys", arr))
+	arr.Noalg = true
 	arr = typ(TARRAY)
 	arr.Type = valtype
 	arr.Bound = BUCKETSIZE