[PATCH] [InstCombine] call SimplifyICmpInst with correct context

[Jingyue Wu]

IIUC, your proposed solution would essentially prevent `SimplifyICmpInst` from leveraging any `@llvm.assume` because `computeKnownBitsFromAssume` does nothing when `Q.CxtI == nullptr`.

It sounds an overkill to me, because `SimplifyICmpInst` does want to leverage `@llvm.assume`s that don't use the compare instruction. For instance,

  b = (a > 1);
  __builtin_assume(b);
  return a > 0;

it's desirable to replace the second compare `a > 0` with `true` while keeping `__builtin_assume(b)` just in case we need `__builtin_assume(b)` in the future.

  b = (a > 1);
  __builtin_assume(b);
  return true;

and the current patch can do this.

What the current patch prevents is `SimplifyICmpInst` leveraging an assume to simplify its ephemeral values (see `isEphemeralValueOf` <http://llvm.org/docs/doxygen/html/ValueTracking_8cpp_source.html#l00372>). That ensures LLVM won't remove `__builtin_assume(b)` because `__builtin_assume(b)` is an ephemeral value of `b`.


http://reviews.llvm.org/D10695

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