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

[Paweł Bylica]

In http://reviews.llvm.org/D4978#140660, @sanjoy wrote:

> For a left shift, I think `HiResult` can be `HiInput << ShiftAmt | BEXTR LoInput, Start = (WordSize - ShiftAmt), Len = ShiftAmt`.  This will do the right thing for `ShiftAmt = 0`.
>
> I think many x86 CPUs do not support `BEXTR` so either this will have be a target dependent thing, or have to be a pattern that the DAG combiner will fold into a `BEXTR`.
>
> In any case, this is very minor.


`BEXTR` would be useful only if shift amount is constant. I think this case is opposite.


http://reviews.llvm.org/D4978

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