cmd/compile: use a buffered channel for the function queue

Updates #20307

With -c=2:

name        old time/op       new time/op       delta
Template          140ms ± 3%        139ms ± 4%  -1.06%  (p=0.003 n=50+50)
Unicode          81.1ms ± 4%       81.9ms ± 4%  +0.96%  (p=0.006 n=50+49)
GoTypes           375ms ± 3%        374ms ± 3%    ~     (p=0.094 n=48+48)
Compiler          1.69s ± 2%        1.68s ± 2%  -0.41%  (p=0.004 n=49+48)
SSA               3.05s ± 1%        3.05s ± 2%    ~     (p=0.953 n=47+49)
Flate            86.3ms ± 2%       85.9ms ± 2%  -0.49%  (p=0.011 n=49+48)
GoParser         99.5ms ± 3%       99.3ms ± 3%    ~     (p=0.394 n=48+49)
Reflect           262ms ± 3%        261ms ± 3%    ~     (p=0.354 n=47+49)
Tar              81.4ms ± 3%       79.7ms ± 4%  -1.98%  (p=0.000 n=47+50)
XML               133ms ± 3%        133ms ± 3%    ~     (p=0.992 n=50+49)
[Geo mean]        236ms             235ms       -0.36%

name        old user-time/op  new user-time/op  delta
Template          249ms ± 5%        242ms ± 7%  -2.61%  (p=0.000 n=48+50)
Unicode           111ms ± 4%        111ms ± 6%    ~     (p=0.407 n=46+47)
GoTypes           753ms ± 2%        748ms ± 3%  -0.65%  (p=0.010 n=48+50)
Compiler          3.28s ± 2%        3.27s ± 2%  -0.40%  (p=0.026 n=49+47)
SSA               7.03s ± 2%        7.01s ± 3%    ~     (p=0.154 n=45+50)
Flate             154ms ± 3%        154ms ± 3%    ~     (p=0.306 n=49+49)
GoParser          180ms ± 4%        179ms ± 4%    ~     (p=0.148 n=48+48)
Reflect           427ms ± 2%        428ms ± 3%    ~     (p=0.502 n=46+49)
Tar               142ms ± 5%        135ms ± 9%  -4.83%  (p=0.000 n=46+50)
XML               247ms ± 3%        247ms ± 4%    ~     (p=0.921 n=49+49)
[Geo mean]        426ms             422ms       -0.92%


Change-Id: I4746234439ddb9a7e5840fc783b8857da6a4a680
Reviewed-on: https://go-review.googlesource.com/43110
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -272,7 +272,7 @@ func compileFunctions() {
 			})
 		}
 		var wg sync.WaitGroup
-		c := make(chan *Node)
+		c := make(chan *Node, nBackendWorkers)
 		for i := 0; i < nBackendWorkers; i++ {
 			wg.Add(1)
 			go func(worker int) {