cmd/compile/internal/gc: remove remaining Nod(OXXX, ...)

cmd/compile/internal/gc: remove remaining Nod(OXXX, ...)

Remove almost all the remaining Nod(OXXX, ... ) uses. The performance
change is due entirely to the changes to func temp(*Type). The other
cleanups have no effect, as expected.

I'll address the remaining Nod(OXXX, ...) uses in a followup CL as they
are very sensitive to change.

lucky(~/go/src/cmd/compile) % benchstat /tmp/{old,new}.txt
name      old time/op    new time/op    delta
Template     391ms ± 6%     385ms ± 6%    ~     (p=0.127 n=19+20)
GoTypes      1.27s ± 2%     1.27s ± 2%    ~     (p=0.172 n=19+19)
Compiler     6.17s ± 2%     6.15s ± 2%    ~     (p=0.647 n=19+20)

name      old alloc/op   new alloc/op   delta
Template    63.7MB ± 0%    63.4MB ± 0%  -0.35%  (p=0.000 n=16+20)
GoTypes      219MB ± 0%     218MB ± 0%  -0.38%  (p=0.000 n=20+20)
Compiler     980MB ± 0%     976MB ± 0%  -0.38%  (p=0.000 n=20+20)

name      old allocs/op  new allocs/op  delta
Template      586k ± 0%      584k ± 0%  -0.30%  (p=0.000 n=20+20)
GoTypes      1.80M ± 0%     1.79M ± 0%  -0.31%  (p=0.000 n=20+20)
Compiler     7.74M ± 0%     7.71M ± 0%  -0.34%  (p=0.000 n=20+20)

Change-Id: Ie21a5443c33a23ce30f987bdddec9fe350365d35
Reviewed-on: https://go-review.googlesource.com/21017
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>