[cfgcleanup] MIPS BE: wrong code generates under "-Os -mbranch-cost=1"

[xiangzhai]

Hi fellows,

As PR83496 mentioned:
Testcase is wrongly lowering to:

	.file	1 "gcc-pr83496.c"
	.section .mdebug.abi64
	.previous
	.nan	legacy
	.module	fp=64
	.module	oddspreg
	.abicalls
	.text
	.align	2
	.globl	mytest
	.set	nomips16
	.set	nomicromips
	.ent	mytest
	.type	mytest, @function
mytest:
	.frame	$sp,0,$31		# vars= 0, regs= 0/0, args= 0, gp= 0
	.mask	0x00000000,0
	.fmask	0x00000000,0
	.set	noreorder
	.set	nomacro
	lw	$3,8($4)
	li	$2,1			# 0x1
	beq	$3,$2,.L3
	nop # <-- it should say "li v0,1" instead of nop

	lw	$3,0($4)
	slt	$3,$3,2
	beq	$3,$0,.L5
	nop

	ld	$2,16($4)
	ld	$2,0($2)
	sltu	$3,$5,$2
	bne	$3,$0,.L5
	nop

	sltu	$2,$2,$5
	j	$31
	subu	$2,$0,$2

.L3:
	j	$31
	li	$2,-1			# 0xffffffffffffffff

.L5:
	j	$31
	li	$2,1			# 0x1

	.set	macro
	.set	reorder
	.end	mytest
	.size	mytest, .-mytest
	.section	.text.startup,"ax",@progbits
	.align	2
	.globl	main
	.set	nomips16
	.set	nomicromips
	.ent	main
	.type	main, @function
main:
	.frame	$sp,0,$31		# vars= 0, regs= 0/0, args= 0, gp= 0
	.mask	0x00000000,0
	.fmask	0x00000000,0
	.set	noreorder
	.set	nomacro
	j	$31
	move	$2,$0

	.set	macro
	.set	reorder
	.end	main
	.size	main, .-main
	.ident	"GCC: (GNU) 5.5.0 20171010 (LLVM China GCC 5.5-2018.01)"

GCC 8.x still reproducible the key point might be bb_is_just_return:

commit 01b8ebb6e5940eca6d158dc793424a6580c90998
Author: segher <segher@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Wed May 4 20:54:47 2016 +0000

    cfgcleanup: Fold jumps and conditional branches with returns
    
    This patch makes cfgcleanup optimize jumps to returns.  There are three
    cases this handles:
    
    -- A jump to a return; this is simplified to just that return.
    -- A conditional branch to a return; simplified to a conditional return.
    -- A conditional branch that falls through to a return.  This is simplified
       to a conditional return (with the condition inverted), falling through
       to a jump to the original destination.  That jump can then be optimized
       further, as usual.
    
    This handles all cases the current function.c does, and a few it misses.
    
    
            * cfgcleanup.c (bb_is_just_return): New function.
            (try_optimize_cfg): Simplify jumps to return, branches to return,
            and branches around return.

Regards,
Leslie Zhai