cmd/compile: redundant branch

[randall77]

func f(x interface{}) bool {
	if x == nil {
		return false
	}
	switch x.(type) {
	case bool:
		return true
	default:
		return false
	}
}

Compiles to

	0x0000 00000 (/Users/khr/go/tmp4.go:3)	MOVQ	"".x+8(SP), AX
	0x0005 00005 (/Users/khr/go/tmp4.go:4)	TESTQ	AX, AX
	0x0008 00008 (/Users/khr/go/tmp4.go:4)	JEQ	47
	0x000a 00010 (/Users/khr/go/tmp4.go:7)	JEQ	41
        ...

That second branch is useless. We should get rid of it somehow.