cmd/compile: teach compiler that uint(x) > 0

[josharian]

func isNegative(x uint) bool {
	if x < 0 {
		return true
	}
	return false
}

I was surprised to see that this compiles to:

	0x0000 00000 (x.go:4)	MOVQ	"".x+8(SP), AX
	0x0005 00005 (x.go:4)	TESTQ	AX, AX
	0x0008 00008 (x.go:4)	JCC	16
	0x000a 00010 (x.go:5)	MOVB	$1, "".~r1+16(SP)
	0x000f 00015 (x.go:5)	RET
	0x0010 00016 (x.go:8)	MOVB	$0, "".~r1+16(SP)
	0x0015 00021 (x.go:8)	RET

I expected just return false.

Noticed while poking around at #36346. Not sure how often this would fire in real code.

[josharian]

When I started writing rewrite rules to fix these, I realized I've been here before. Oops. Duplicate of #21439.