cmd/compile: unneeded panicshift for bounded shift amount

[CAFxX]

Tested on 1.15.3 and tip.

Foo contains an unnecessary panicshift call, whereas Bar does not:

func Foo(x int) byte {
    if x >= 0 { 
        return byte(1) << (x % 8)
    }
    return 0
}

func Bar(x uint) byte {
    return byte(1) << (x % 8)
}

godbolt link: https://godbolt.org/z/qhTqYM

[cuonglm]

It seems because we rewrite Mod64 early before prove pass, so it's too late to realize the optimization.