[SDAG] Simplify is-power-of-2 codegen (#72275)

When x is not known to be nonzero, ctpop(x) == 1 is expanded to

    x != 0 && (x & (x - 1)) == 0

resulting in codegen like

    leal    -1(%rdi), %eax
    testl   %eax, %edi
    sete    %cl
    testl   %edi, %edi
    setne   %al
    andb    %cl, %al

But another expression that works is

    (x ^ (x - 1)) > x - 1

which has nicer codegen:

    leal    -1(%rdi), %eax
    xorl    %eax, %edi
    cmpl    %eax, %edi
    seta    %al