errors: make errors.Trace mid-stack inlineable

[lysu]

go 1.12 with more aggressive inline strategy

https://tip.golang.org/doc/go1.12

More functions are now eligible for inlining by default, including functions that do nothing but call another function.

so error.Trace can be inlined in 1.12, let nil check to Trace make less low level method calling

goos: linux
goarch: amd64
pkg: github.com/pingcap/errors
BenchmarkOld-8   	100000000	        18.3 ns/op
BenchmarkNew-8   	1000000000	         2.26 ns/op
PASS

with default -l, go1.12 also can inline Trace,

go build -gcflags '-m=2 -N' 
# github.com/pingcap/errors/m/x
./m.go:14:6: can inline f2 as: func() error { return errors.New("") }
./m.go:15:19: inlining call to errors.New func(string) error { return error(&errors.fundamental literal) }
./m.go:15:19: inlining call to errors.callers func() *errors.stack { return errors.callersSkip(int(4)) }
./m.go:10:6: cannot inline f1: function too complex: cost 146 exceeds budget 80
./m.go:11:25: inlining call to f2 func() error { return errors.New("") }
./m.go:11:25: inlining call to errors.New func(string) error { return error(&errors.fundamental literal) }
./m.go:11:25: inlining call to errors.callers func() *errors.stack { return errors.callersSkip(int(4)) }
./m.go:11:22: inlining call to errors.Trace2 func(error) error { if errors.err == nil { return nil }; return errors.AddStack(errors.err) }

on another side in go1.11.3

go build -gcflags '-m=2 -l=3 -N'  
# github.com/pingcap/errors/m/x
./m.go:14:6: cannot inline f2: function too complex: cost 84 exceeds budget 80
./m.go:10:6: cannot inline f1: function too complex: cost 165 exceeds budget 80
./m.go:5:6: cannot inline main: function too complex: cost 89 exceeds budget 80

and need -l=4 to make mid-stack inline, so this isn't work in version < 1.12

this modification is very similar to https://go-review.googlesource.com/c/go/+/156362/

This change is 

[gregwebs]

What happens if the nil check is placed in AddStack?

[lysu]

@gregwebs I have do some test, it seems AddStack can not be inline

package main

import "github.com/pingcap/errors"

func main() {
	e := f1()
	e = e
}

func f1() error {
	return errors.AddStack2(nil)
}

go build -gcflags '-m=2 -N' 
# github.com/pingcap/errors/m/x
./m.go:10:6: can inline f1 as: func() error { return errors.AddStack2(nil) }
./m.go:5:6: can inline main as: func() { e := f1(); e = e }
./m.go:6:9: inlining call to f1 func() error { return errors.AddStack2(nil) }

so, it need jump into the method to check if-condition, so little slow 0.3 ns/op

[jackysp]

LGTM

[ngaut]

LGTM