[mlir][MemRef] Fix SubViewOp canonicalization when a subset of unit-dims are dropped.

The canonical type of the result of the `memref.subview` needs to make
sure that the previously dropped unit-dimensions are the ones dropped
for the canonicalized type as well. This means the generic
`inferRankReducedResultType` cannot be used. Instead the current
dropped dimensions need to be querried and the same need to be dropped.

Reviewed By: nicolasvasilache, ThomasRaoux

Differential Revision: https://reviews.llvm.org/D114751