[InstSimplify] Fold min/max intrinsic based on icmp of operands

This is a reboot of D84655, now performing the inner icmp
simplification query without undef folds.

It should be possible to handle the current foldMinMaxSharedOp()
fold based on this, by moving the logic into icmp of min/max instead,
making it more general. We can't drop the folds for constant operands,
because those also allow undef, which we exclude here.

The tests use assumes for exhaustive coverage, and have a few
more examples of misc folds we get based on icmp simplification.

Differential Revision: https://reviews.llvm.org/D85929