[PATCH] Fix generic shift expansion when shift amount is 0

[Paweł Bylica]

In http://reviews.llvm.org/D4978#136194, @sanjoy wrote:

> Shallow, drop-by comment: does it make sense to generate a DAG that can be combined into a `BEXTR` when possible?


Can you explain more?


http://reviews.llvm.org/D4978

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