cmd/compile: optimize functions into boolean expressions

[dsnet]

Using 587b803

In http://golang.org/cl/32122, I optimized a switch-based implementation to be a boolean expression. The two implementations below are semantically equivalent:

func ValidRune1(r rune) bool {
    switch {
    case r < 0:
        return false
    case surrogateMin <= r && r <= surrogateMax:
        return false
    case r > MaxRune:
        return false
    }
    return true
}

func ValidRune2(r rune) bool {
    return (0 <= r && r < surrogateMin) || (surrogateMax < r && r <= MaxRune)
}

Testable benchmark: https://play.golang.org/p/KMlv43EVQn
The benchmark results:

benchmark        old ns/op     new ns/op     delta
BenchmarkA-4     16.3          11.9          -26.99%

We could consider having the compiler recognize the pattern of switch (with boolean expression as cases) and all return values are either boolean literals or expressions and have the compiler automatically optimize that into a single larger boolean expression.

\cc @bradfitz @minux @martisch