This CL reworks walkcompare for clarity and concision.
It also makes one significant functional change.
(The functional change is hard to separate cleanly
from the cleanup, so I just did them together.)
When inlining and unrolling an equality comparison
for a small struct or array, compare the elements like:

a[0] == b[0] && a[1] == b[1]

rather than

pa := &a
pb := &b
pa[0] == pb[0] && pa[1] == pb[1]

The result is the same, but taking the address
and working through the indirect
forces the backends to generate less efficient code.

This is only an improvement with the SSA backend.
However, every port but s390x now has a working
SSA backend, and switching to the SSA backend
by default everywhere is a priority for Go 1.8.
It thus seems reasonable to start to prioritize
SSA performance over the old backend.

Updates #15303


Sample code:

type T struct {
	a, b int8
}

func g(a T) bool {
	return a == T{1, 2}
}


SSA before:

"".g t=1 size=80 args=0x10 locals=0x8
	0x0000 00000 (badeq.go:7)	TEXT	"".g(SB), $8-16
	0x0000 00000 (badeq.go:7)	SUBQ	$8, SP
	0x0004 00004 (badeq.go:7)	FUNCDATA	$0, gclocals·23e8278e2b69a3a75fa59b23c49ed6ad(SB)
	0x0004 00004 (badeq.go:7)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0004 00004 (badeq.go:8)	MOVBLZX	"".a+16(FP), AX
	0x0009 00009 (badeq.go:8)	MOVB	AL, "".autotmp_0+6(SP)
	0x000d 00013 (badeq.go:8)	MOVBLZX	"".a+17(FP), AX
	0x0012 00018 (badeq.go:8)	MOVB	AL, "".autotmp_0+7(SP)
	0x0016 00022 (badeq.go:8)	MOVB	$0, "".autotmp_1+4(SP)
	0x001b 00027 (badeq.go:8)	MOVB	$1, "".autotmp_1+4(SP)
	0x0020 00032 (badeq.go:8)	MOVB	$2, "".autotmp_1+5(SP)
	0x0025 00037 (badeq.go:8)	MOVBLZX	"".autotmp_0+6(SP), AX
	0x002a 00042 (badeq.go:8)	MOVBLZX	"".autotmp_1+4(SP), CX
	0x002f 00047 (badeq.go:8)	CMPB	AL, CL
	0x0031 00049 (badeq.go:8)	JNE	70
	0x0033 00051 (badeq.go:8)	MOVBLZX	"".autotmp_0+7(SP), AX
	0x0038 00056 (badeq.go:8)	CMPB	AL, $2
	0x003a 00058 (badeq.go:8)	SETEQ	AL
	0x003d 00061 (badeq.go:8)	MOVB	AL, "".~r1+24(FP)
	0x0041 00065 (badeq.go:8)	ADDQ	$8, SP
	0x0045 00069 (badeq.go:8)	RET
	0x0046 00070 (badeq.go:8)	MOVB	$0, AL
	0x0048 00072 (badeq.go:8)	JMP	61

SSA after:

"".g t=1 size=32 args=0x10 locals=0x0
	0x0000 00000 (badeq.go:7)	TEXT	"".g(SB), $0-16
	0x0000 00000 (badeq.go:7)	NOP
	0x0000 00000 (badeq.go:7)	NOP
	0x0000 00000 (badeq.go:7)	FUNCDATA	$0, gclocals·23e8278e2b69a3a75fa59b23c49ed6ad(SB)
	0x0000 00000 (badeq.go:7)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0000 00000 (badeq.go:8)	MOVBLZX	"".a+8(FP), AX
	0x0005 00005 (badeq.go:8)	CMPB	AL, $1
	0x0007 00007 (badeq.go:8)	JNE	25
	0x0009 00009 (badeq.go:8)	MOVBLZX	"".a+9(FP), CX
	0x000e 00014 (badeq.go:8)	CMPB	CL, $2
	0x0011 00017 (badeq.go:8)	SETEQ	AL
	0x0014 00020 (badeq.go:8)	MOVB	AL, "".~r1+16(FP)
	0x0018 00024 (badeq.go:8)	RET
	0x0019 00025 (badeq.go:8)	MOVB	$0, AL
	0x001b 00027 (badeq.go:8)	JMP	20


Change-Id: I120185d58012b7bbcdb1ec01225b5b08d0855d86
Reviewed-on: https://go-review.googlesource.com/22277
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>