[PatternMatch] Make m_c_ICmp swap the predicate (PR42801)

This addresses https://bugs.llvm.org/show_bug.cgi?id=42801.
The m_c_ICmp() matcher is changed to provide the swapped predicate
if the operands are swapped.

Existing uses of m_c_ICmp() fall in one of two categories: Working
on equality predicates only, where swapping is irrelevant.
Or performing a manual swap, in which case this patch removes it.

The only exception is the foldICmpWithLowBitMaskedVal() fold, which
does not swap the predicate, and instead reasons about whether
a swap occurred or not for each predicate. Getting the swapped
predicate allows us to merge the logic for pairs of predicates,
instead of duplicating it.

Differential Revision: https://reviews.llvm.org/D72976