[PATCH] DAGCombiner: Add another rotate pattern.

[David Majnemer]

It is not uncommon to see code like the following:

(x << n) | (x >> (-n & 31))

This is convenient because it properly handles the cases where n is zero
whereas:

(x << n) | (x >> (32 - n))

suffers from undefined behavior in that case.

The attached patch teaches SelectionDAG's DAGCombiner how to fold the first
pattern into a rotation.
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