cmd/compile: don’t generate a new Node for convas

This removes about 3% of the Nodes allocated
while compiling std+cmd.

Passes toolstash -cmp.

name       old time/op     new time/op     delta
Template       320ms ± 3%      316ms ± 5%    ~           (p=0.063 n=21+23)
Unicode        162ms ± 9%      161ms ± 6%    ~           (p=0.788 n=25+25)
GoTypes        1.03s ± 4%      1.03s ± 4%    ~           (p=0.929 n=24+25)
Compiler       4.99s ± 3%      4.95s ± 2%  -0.84%        (p=0.011 n=25+23)
MakeBash       40.3s ± 1%      40.3s ± 1%    ~           (p=0.468 n=24+24)

name       old alloc/op    new alloc/op    delta
Template      57.3MB ± 0%     57.0MB ± 0%  -0.51%        (p=0.000 n=25+23)
Unicode       41.1MB ± 0%     41.0MB ± 0%  -0.27%        (p=0.000 n=25+24)
GoTypes        191MB ± 0%      190MB ± 0%  -0.46%        (p=0.000 n=25+25)
Compiler       839MB ± 0%      834MB ± 0%  -0.62%        (p=0.000 n=24+24)

name       old allocs/op   new allocs/op   delta
Template        500k ± 0%       498k ± 0%  -0.42%        (p=0.000 n=25+25)
Unicode         400k ± 0%       399k ± 0%  -0.22%        (p=0.000 n=24+25)
GoTypes        1.50M ± 0%      1.49M ± 0%  -0.41%        (p=0.000 n=23+25)
Compiler       6.04M ± 0%      6.00M ± 0%  -0.59%        (p=0.000 n=25+25)

Change-Id: I7d3f177d1ab4a75a4c047fa465f2eee38747603f
Reviewed-on: https://go-review.googlesource.com/21178
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -774,9 +774,9 @@ opswitch:
 		}
 
 		if n.Left != nil && n.Right != nil {
-			r := convas(Nod(OAS, n.Left, n.Right), init)
-			r.Dodata = n.Dodata
-			n = r
+			dd := n.Dodata
+			n = convas(n, init)
+			n.Dodata = dd
 			n = applywritebarrier(n)
 		}