[PATCH] D133369: [InstCombine] fold add+negate through select into sub

[Sanjay Patel via Phabricator via llvm-commits]

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This transform came up as a potential DAGCombine in D133282 <https://reviews.llvm.org/D133282>, so I wanted to see how it escaped in IR too.

We do general folds in InstCombiner::SimplifySelectsFeedingBinaryOp() by checking if either arm of a select simplifies when the trailing binop is threaded into the select.

So as long as one side simplifies, it's a good fold to combine a negate and add into 1 subtract.

This is an example with a zero arm in the select:
https://alive2.llvm.org/ce/z/Hgu_Tj

And this models the tests with a cancelling 'not' op: 
https://alive2.llvm.org/ce/z/BuzVV_


https://reviews.llvm.org/D133369

Files:
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/add.ll

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