cmd/compile: turn on DWARF locations lists for ssa vars

This changes the default setting for -dwarflocationlists
from false to true, removes the flag from ssa/debug_test.go,
and updates runtime/runtime-gdb_test.go to match a change
in debugging output for composite variables.

Current benchmarks (perflock, -count 10)

benchstat -geomean before.log after.log
name        old time/op     new time/op     delta
Template        175ms ± 0%      182ms ± 1%   +3.68%  (p=0.000 n=8+9)
Unicode        82.0ms ± 2%     82.8ms ± 1%   +0.96%  (p=0.019 n=9+9)
GoTypes         590ms ± 1%      611ms ± 1%   +3.42%  (p=0.000 n=9+10)
Compiler        2.85s ± 0%      2.95s ± 1%   +3.60%  (p=0.000 n=9+10)
SSA             6.42s ± 1%      6.70s ± 1%   +4.31%  (p=0.000 n=10+9)
Flate           113ms ± 2%      117ms ± 1%   +3.11%  (p=0.000 n=10+9)
GoParser        140ms ± 1%      145ms ± 1%   +3.47%  (p=0.000 n=10+9)
Reflect         384ms ± 0%      398ms ± 1%   +3.56%  (p=0.000 n=8+9)
Tar             165ms ± 1%      171ms ± 1%   +3.33%  (p=0.000 n=9+9)
XML             207ms ± 2%      214ms ± 1%   +3.41%  (p=0.000 n=9+9)
StdCmd          11.8s ± 2%      12.4s ± 2%   +4.41%  (p=0.000 n=10+9)
[Geo mean]      489ms           506ms        +3.38%

name        old user-ns/op  new user-ns/op  delta
Template         247M ± 4%       254M ± 4%   +2.76%  (p=0.040 n=10+10)
Unicode          118M ±16%       121M ±11%     ~     (p=0.364 n=10+10)
GoTypes          805M ± 2%       824M ± 2%   +2.37%  (p=0.003 n=9+8)
Compiler        3.92G ± 2%      4.01G ± 2%   +2.20%  (p=0.001 n=9+9)
SSA             9.63G ± 4%     10.00G ± 2%   +3.81%  (p=0.000 n=10+9)
Flate            155M ±10%       154M ± 7%     ~     (p=0.718 n=9+10)
GoParser         184M ±11%       190M ± 7%     ~     (p=0.220 n=10+9)
Reflect          506M ± 4%       528M ± 2%   +4.27%  (p=0.000 n=10+10)
Tar              224M ± 4%       227M ± 5%     ~     (p=0.207 n=10+9)
XML              272M ± 7%       286M ± 4%   +5.23%  (p=0.010 n=10+9)
[Geo mean]       489M            502M        +2.76%

name        old text-bytes  new text-bytes  delta
HelloSize        672k ± 0%       672k ± 0%     ~     (all equal)
CmdGoSize       7.21M ± 0%      7.21M ± 0%     ~     (all equal)
[Geo mean]      2.20M           2.20M        +0.00%

name        old data-bytes  new data-bytes  delta
HelloSize       9.88k ± 0%      9.88k ± 0%     ~     (all equal)
CmdGoSize        248k ± 0%       248k ± 0%     ~     (all equal)
[Geo mean]      49.5k           49.5k        +0.00%

name        old bss-bytes   new bss-bytes   delta
HelloSize        125k ± 0%       125k ± 0%     ~     (all equal)
CmdGoSize        144k ± 0%       144k ± 0%     ~     (all equal)
[Geo mean]       135k            135k        +0.00%

name        old exe-bytes   new exe-bytes   delta
HelloSize       1.10M ± 0%      1.30M ± 0%  +17.82%  (p=0.000 n=10+10)
CmdGoSize       11.6M ± 0%      13.5M ± 0%  +16.90%  (p=0.000 n=10+10)
[Geo mean]      3.57M           4.19M       +17.36%

Change-Id: I250055813cadd25cebee8da1f9a7f995a6eae432
Reviewed-on: https://go-review.googlesource.com/100738
Reviewed-by: Heschi Kreinick <heschi@google.com>

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

@@ -135,10 +135,10 @@ func TestNexting(t *testing.T) {
 		t.Skip(skipReasons[:len(skipReasons)-2])
 	}
 
-	optFlags := "-dwarflocationlists"
+	optFlags := "" // Whatever flags are needed to test debugging of optimized code.
 	if !*useDelve && !*inlines {
 		// For gdb (default), disable inlining so that a compiler test does not depend on library code.
-		// TODO: This may not be necessary with 1.10 and later.
+		// TODO: Technically not necessary in 1.10, but it causes a largish regression that needs investigation.
 		optFlags += " -l"
 	}