[mlir] Add min/max operations to Standard.

[RFC: Add min/max ops](https://llvm.discourse.group/t/rfc-add-min-max-operations/4353)

I was following the naming style for Arith dialect in
https://reviews.llvm.org/D110200,
i.e. similar to DivSIOp and DivUIOp I defined MaxSIOp, MaxUIOp.

When Arith PR is landed, I will migrate these ops as well.

Differential Revision: https://reviews.llvm.org/D110540