[InstSimplify] fold redundant select of min/max

This is similar to the existing simplify:
max (max X, Y), X --> max X, Y
...but the select condition can be one of
several predicates as shown in the tests.

The tests cycle through all 10 icmp preds for
each min/max variant with 4 swapped operand
patterns each (and the min/max operands are
commuted in every other test within those).

Some Alive2 examples to verify:
https://alive2.llvm.org/ce/z/lCAQm4
https://alive2.llvm.org/ce/z/kzxVXC