[PATCH] Fold fcmp in cases where value is provably non-negative.

[Arch D. Robison]

Hi reames, hfinkel,

This patch folds fcmp in some cases of interest in Julia.  The patch adds a function CannotBeOrderedLessThanZero that returns true if a value is provably not less than zero.  I.e. the function returns true if the value is provably -0, +0, positive, or a NaN.  The patch extends InstructionSimplify.cpp to fold instances of fcmp where:

  -  the predicate is olt or uge
  -  the first operand is provably not less than zero
  -  the second operand is zero

The motivation for handling these cases optimizing away domain checks for sqrt in Julia for common idioms such as sqrt(x*x+y*y)..  

- Arch

http://reviews.llvm.org/D6972

Files:
  include/llvm/Analysis/ValueTracking.h
  lib/Analysis/InstructionSimplify.cpp
  lib/Analysis/ValueTracking.cpp
  test/Transforms/InstSimplify/floating-point-compare.ll

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