[mlir][Arith] Remove expansions of integer min and max ops

As of several months ago, both ArithToLLVM and ArithToSPIRV have
native support for integer min and max operations. Since these are all
the targets available in MLIR core, the need to "expand" arith.minui,
arith.minsi, arith,maxsi, and arith.manxui to more primitive
operations is to longer present.

Therefore, the expanding of integer min and max operations in Arith,
while correct, is likely to lead to performance loss by way of
misoptimization further down the line, and is no longer needed for
anyone's correctness.

This change may break downstream tests, but will not affect the
semantics of MLIR programs.

arith.minf and arith.maxf have a lot of underlying complexity due to
the many different possible NaN and signed zero semantics available on
various platforms, and so removing their expansion is left to a future
commit.

Reviewed By: ThomasRaoux, Mogball

Differential Revision: https://reviews.llvm.org/D140856

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