[PATCH][DAGCombiner] Distribute TRUNC through AND in rotation amount

[Adam Nemet]

This is already done for shifts.  Allow it for rotations as well. E.g.:

   (rotl:i32 x, (trunc (and y, 31))) -> (rotl:i32 x, (and (trunc y), 31))

Use the newly factored-out distributeTruncateThroughAnd.

With this patch and some X86.td tweaks we should be able to remove redundant
masking of the rotation amount like in the example above.  HW implicitly
performs this masking.

The testcase will be added as part of the X86 patch.

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