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

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 slightly less code, do that.

No behavior change.

Differential Revision: https://reviews.llvm.org/D100656