[PATCH] Teach ScalarEvolution to exploit min and max expressions.

[Sanjoy Das]

Hi atrick, reames, nlewycky, hfinkel,

Teach ScalarEvolution to exploit min and max expressions when proving `isKnownPredicate`.

The motivation for this change is to optimize away checks in loops like this:

  limit = min(t, len)
  for (i = 0 to limit)
    if (i >= len || i < 0) throw_array_of_of_bounds();
    a[i] = ...

http://reviews.llvm.org/D6635

Files:
  lib/Analysis/ScalarEvolution.cpp
  test/Transforms/IndVarSimplify/backedge-on-min-max.ll

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