Use SETcc instructions instead of Jcc to generate boolean values.
This generates shorter, jump-free code, which may in turn enable other
peephole optimizations.

For example, given

func f(i, j int) bool {
	return i == j
}

Before

"".f t=1 size=32 value=0 args=0x18 locals=0x0
	0x0000 00000 (x.go:3)	TEXT	"".f(SB), $0-24
	0x0000 00000 (x.go:3)	FUNCDATA	$0, gclocals·b4c25e9b09fd0cf9bb429dcefe91c353(SB)
	0x0000 00000 (x.go:3)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0000 00000 (x.go:4)	MOVQ	"".i+8(FP), BX
	0x0005 00005 (x.go:4)	MOVQ	"".j+16(FP), BP
	0x000a 00010 (x.go:4)	CMPQ	BX, BP
	0x000d 00013 (x.go:4)	JEQ	21
	0x000f 00015 (x.go:4)	MOVB	$0, "".~r2+24(FP)
	0x0014 00020 (x.go:4)	RET
	0x0015 00021 (x.go:4)	MOVB	$1, "".~r2+24(FP)
	0x001a 00026 (x.go:4)	JMP	20

After

"".f t=1 size=32 value=0 args=0x18 locals=0x0
	0x0000 00000 (x.go:3)	TEXT	"".f(SB), $0-24
	0x0000 00000 (x.go:3)	FUNCDATA	$0, gclocals·b4c25e9b09fd0cf9bb429dcefe91c353(SB)
	0x0000 00000 (x.go:3)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0000 00000 (x.go:4)	MOVQ	"".i+8(FP), BX
	0x0005 00005 (x.go:4)	MOVQ	"".j+16(FP), BP
	0x000a 00010 (x.go:4)	CMPQ	BX, BP
	0x000d 00013 (x.go:4)	SETEQ	"".~r2+24(FP)
	0x0012 00018 (x.go:4)	RET

regexp benchmarks, best of 12 runs:

benchmark                                 old ns/op      new ns/op      delta
BenchmarkNotOnePassShortB                 782            733            -6.27%
BenchmarkLiteral                          180            171            -5.00%
BenchmarkNotLiteral                       2855           2721           -4.69%
BenchmarkMatchHard_32                     2672           2557           -4.30%
BenchmarkMatchHard_1K                     80182          76732          -4.30%
BenchmarkMatchEasy1_32M                   76440180       73304748       -4.10%
BenchmarkMatchEasy1_32K                   68798          66350          -3.56%
BenchmarkAnchoredLongMatch                482            465            -3.53%
BenchmarkMatchEasy1_1M                    2373042        2292692        -3.39%
BenchmarkReplaceAll                       2776           2690           -3.10%
BenchmarkNotOnePassShortA                 1397           1360           -2.65%
BenchmarkMatchClass_InRange               3842           3742           -2.60%
BenchmarkMatchEasy0_32                    125            122            -2.40%
BenchmarkMatchEasy0_32K                   11414          11164          -2.19%
BenchmarkMatchEasy0_1K                    668            654            -2.10%
BenchmarkAnchoredShortMatch               260            255            -1.92%
BenchmarkAnchoredLiteralShortNonMatch     164            161            -1.83%
BenchmarkOnePassShortB                    623            612            -1.77%
BenchmarkOnePassShortA                    801            788            -1.62%
BenchmarkMatchClass                       4094           4033           -1.49%
BenchmarkMatchEasy0_32M                   14078800       13890704       -1.34%
BenchmarkMatchHard_32K                    4095844        4045820        -1.22%
BenchmarkMatchEasy1_1K                    1663           1643           -1.20%
BenchmarkMatchHard_1M                     131261708      129708215      -1.18%
BenchmarkMatchHard_32M                    4210112412     4169292003     -0.97%
BenchmarkMatchMedium_32K                  2460752        2438611        -0.90%
BenchmarkMatchEasy0_1M                    422914         419672         -0.77%
BenchmarkMatchMedium_1M                   78581121       78040160       -0.69%
BenchmarkMatchMedium_32M                  2515287278     2498464906     -0.67%
BenchmarkMatchMedium_32                   1754           1746           -0.46%
BenchmarkMatchMedium_1K                   52105          52106          +0.00%
BenchmarkAnchoredLiteralLongNonMatch      185            185            +0.00%
BenchmarkMatchEasy1_32                    107            107            +0.00%
BenchmarkOnePassLongNotPrefix             505            505            +0.00%
BenchmarkOnePassLongPrefix                147            147            +0.00%

The godoc binary is ~0.12% smaller after this CL.

Updates #5729.

toolstash -cmp passes for all architectures other than amd64 and amd64p32.

Other architectures can be done in follow-up CLs.

Change-Id: I0e167e259274b722958567fc0af83a17ca002da7
Reviewed-on: https://go-review.googlesource.com/2284Reviewed-by: Russ Cox <rsc@golang.org>