cmd/compile/internal/gc: disable logProgs debug flag

Spotted while splunking in the compiler with GOGC=off.

name       old time/op     new time/op     delta
Template       407ms ± 5%      402ms ± 6%     ~           (p=0.301 n=20+20)
GoTypes        1.33s ± 2%      1.29s ± 1%   -3.47%        (p=0.000 n=20+20)
Compiler       6.21s ± 1%      5.91s ± 2%   -4.83%        (p=0.000 n=20+20)

name       old alloc/op    new alloc/op    delta
Template      66.8MB ± 0%     63.9MB ± 0%   -4.46%        (p=0.000 n=19+20)
GoTypes        232MB ± 0%      220MB ± 0%   -5.16%        (p=0.000 n=19+17)
Compiler      1.02GB ± 0%     0.97GB ± 0%   -5.81%        (p=0.000 n=20+20)

name       old allocs/op   new allocs/op   delta
Template        789k ± 0%       708k ± 0%  -10.28%        (p=0.000 n=19+20)
GoTypes        2.49M ± 0%      2.20M ± 0%  -11.57%        (p=0.000 n=20+20)
Compiler       10.8M ± 0%       9.4M ± 0%  -12.82%        (p=0.000 n=20+20)

Change-Id: I76615cab912dde10595ca6ab9979ff6c5f1aec49
Reviewed-on: https://go-review.googlesource.com/20782Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -3654,7 +3654,7 @@ func genssa(f *ssa.Func, ptxt *obj.Prog, gcargs, gclocals *Sym) {
 
 	var valueProgs map[*obj.Prog]*ssa.Value
 	var blockProgs map[*obj.Prog]*ssa.Block
-	const logProgs = true
+	var logProgs = e.log
 	if logProgs {
 		valueProgs = make(map[*obj.Prog]*ssa.Value, f.NumValues())
 		blockProgs = make(map[*obj.Prog]*ssa.Block, f.NumBlocks())