cmd/compile: don't use lookupN for statictmps

cmd/compile: don't use lookupN for statictmps

The names never occur more than once,
so interning the results is counterproductive.

The impact is not very big, but neither is the fix.

name     old time/op     new time/op     delta
Unicode     90.2ms ± 3%     88.3ms ± 5%  -2.10%  (p=0.000 n=94+98)

Change-Id: I1e3a24433db4ae0c9a6e98166568941824ff0779
Reviewed-on: https://go-review.googlesource.com/39193
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>