[DAGCombiner] Slightly improve readability of matchRotateSub

Slightly change the wording in the function comment. Originally, it can be
misunderstood as we turned the input into two subsequent rotates.

Better connect the comment which talks about Mask and the code which used
LoBits.  Renamed variable to MaskLoBits.
[DAGCombiner] Recognize another rotation idiom

This is the new idiom:

  x<<(y&31) | x>>((0-y)&31)

which is recognized as:

  x ROTL (y&31)

The change refines matchRotateSub.  In
Neg & (OpSize - 1) == (OpSize - Pos) & (OpSize - 1), if Pos is
Pos' & (OpSize - 1) we can just use Pos' instead of Pos.
[DAGCombiner] Distribute TRUNC through AND in rotation amount

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.