[LoopIdiomRecognize] Teach detectShiftUntilZeroIdiom to recognize loops where...

[LoopIdiomRecognize] Teach detectShiftUntilZeroIdiom to recognize loops where the counter is decrementing.

This adds support for loops like

unsigned clz(unsigned x) {
    unsigned w = sizeof (x) * CHAR_BIT;
    while (x) {
        w--;
        x >>= 1;
    }

    return w;
}

and

unsigned clz(unsigned x) {
    unsigned w = sizeof (x) * CHAR_BIT - 1;
    while (x >>= 1) {
        w--;
    }

    return w;
}

To support these we look for add x, -1 as well as add x, 1 that
we already matched. If the value was -1 we need to subtract from
the initial counter value instead of adding to it.

Fixes PR48404.

Differential Revision: https://reviews.llvm.org/D92745