rdar://12329730 (defect 2)

Enhancement to InstCombine. Try to catch this opportunity:
  
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 ((X^C1) >> C2) ^ C3  => (X>>C2) ^ ((C1>>C2)^C3)
  where the subexpression "X ^ C1" has more than one uses, and
  "(X^C1) >> C2" has single use. 
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 Reviewed by Nadav (with minor change per his request).

llvm-svn: 168615