[PATCH] Carry facts about nullness and undef across GC relocation

[Philip Reames]

Hi sanjoy,

This change implements four basic optimizations:
- If a relocated value isn't used, it doesn't need to be relocated.
- If the value being relocated is null, relocation doesn't change that.  (Technically, this might be collector specific.   I don't know of one which it doesn't work for though.)
- If the value being relocated is undef, the relocation is meaningless.
- If the value being relocated was known nonnull, the relocated pointer also isn't null.  (Since it points to the same source language object.)

I outlined other planned work in comments.

http://reviews.llvm.org/D6600

Files:
  include/llvm/IR/Statepoint.h
  lib/Transforms/InstCombine/InstCombineCalls.cpp
  test/Transforms/InstCombine/statepoint.ll

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