[Support] Don't include <algorithm> in MathExtras.h

[thakis]

Summary

MathExtras.h is indirectly included in over 98% of LLVM's
translation units. It currently expands to over 1MB of stuff,
over which far more than half is due to <algorithm>. Since not
using <algorithm> is slighly less code, do that.

No behavior change.

[thakis]

(Found by https://commondatastorage.googleapis.com/chromium-browser-clang/llvm-include-analysis.html – it's unclear how many TUs don't include <algorithm> after this, maybe it's still included in most places through some other common header)

[hans]

slightly different codegen: https://godbolt.org/z/1zebofo98
but probably just as good :)

[stella.stamenova]

This broke several windows bots, for example the MLIR one:

https://lab.llvm.org/buildbot/#/builders/13/builds/6984

[thakis]

In D100656#2695112, @stella.stamenova wrote:

This broke several windows bots, for example the MLIR one:

https://lab.llvm.org/buildbot/#/builders/13/builds/6984

…isn't the fix for that to instead add the include to APInt.h? Why the revert?

[thakis]

Well, trying again in 0b36a33ab894 :)

llvm/include/llvm/Support/MathExtras.h

 //===-- llvm/Support/MathExtras.h - Useful math functions -------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file contains some functions that are useful for math stuff.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_MATHEXTRAS_H
 #define LLVM_SUPPORT_MATHEXTRAS_H
 
 #include "llvm/Support/Compiler.h"
-#include <algorithm>
 #include <cassert>
 #include <climits>
 #include <cmath>
 #include <cstdint>
 #include <cstring>
 #include <limits>
 #include <type_traits>
 
inline int64_t SignExtend64(uint64_t X, unsigned B) {
   assert(B <= 64 && "Bit width out of range.");
   return int64_t(X << (64 - B)) >> (64 - B);
 }
 
 /// Subtract two unsigned integers, X and Y, of type T and return the absolute
 /// value of the result.
 template <typename T>
 std::enable_if_t<std::is_unsigned<T>::value, T> AbsoluteDifference(T X, T Y) {
-  return std::max(X, Y) - std::min(X, Y);
+  return X > Y ? X - Y : Y - X;
 }
 
 /// Add two unsigned integers, X and Y, of type T.  Clamp the result to the
 /// maximum representable value of T on overflow.  ResultOverflowed indicates if
 /// the result is larger than the maximum representable value of type T.
 template <typename T>
 std::enable_if_t<std::is_unsigned<T>::value, T>
 SaturatingAdd(T X, T Y, bool *ResultOverflowed = nullptr) {