MergeICmps does not work for non-eq comparisons

[Kmeakin]

Given a struct like

struct S {
    uint8_t a;
    uint8_t b;
    uint8_t c;
    uint8_t d;
    std::strong_ordering operator<=>(const S&) const = default;
};

MergeICmps is able to optimise the operator== implementation to a single uint32_t load and compare, but is not able to do the same for the other comparison operators

https://godbolt.org/z/Pnqa1cq5q

[vfdff]

candidate MR: https://reviews.llvm.org/D141188

[vfdff]

This issue should remain open, because this only handles the ne case, not other predicates

[Kmeakin]

Reopening: this patch does not fix the other cases (lt, gt, le, ge)

[Kmeakin]

Reopening: this patch does not fix the other cases (lt, gt, le, ge)