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

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

The pattern

    n := Nod(OXXX, nil, nil)
    Nodconst(n, ...)

was a leftover from the C days where n must be heap allocated.

No change in benchmarks, none expected as n escapes anyway.

name      old time/op    new time/op    delta
Template     391ms ± 6%     388ms ± 5%    ~     (p=0.659 n=20+20)
GoTypes      1.27s ± 1%     1.27s ± 2%    ~     (p=0.828 n=18+20)
Compiler     6.16s ± 2%     6.15s ± 1%    ~     (p=0.947 n=20+20)

name      old alloc/op   new alloc/op   delta
Template    63.7MB ± 0%    63.7MB ± 0%    ~     (p=0.414 n=20+20)
GoTypes      219MB ± 0%     219MB ± 0%    ~     (p=0.904 n=20+20)
Compiler     980MB ± 0%     980MB ± 0%  +0.00%  (p=0.007 n=20+19)

name      old allocs/op  new allocs/op  delta
Template      586k ± 0%      586k ± 0%    ~     (p=0.564 n=19+20)
GoTypes      1.80M ± 0%     1.80M ± 0%    ~     (p=0.718 n=20+20)
Compiler     7.74M ± 0%     7.74M ± 0%    ~     (p=0.358 n=20+20)

The reuse of nc in multiple overlapping scopes in walk.go is the worst.

Change-Id: I4ed6a63f7ffbfff68124ad609f6e3a68d95cbbba
Reviewed-on: https://go-review.googlesource.com/21015
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>