[InstCombine] try to narrow min/max intrinsics with constant operand

The constant trunc/ext may not be the optimal pre-condition,
but I think that handles the common cases.

Example of Alive2 proof:
https://alive2.llvm.org/ce/z/sREeLC

This is another step towards canonicalizing to the intrinsics.
Narrowing was identified as source of potential regression for
abs(), so we need to handle this for min/max - see:
https://llvm.org/PR48816

If this is not enough, we could process intrinsics in
the trunc-driven matching in canEvaluateTruncated().