This patch enhances `MemRefType::areTrailingDimsContiguous` to also
handle memrefs with dynamic dimensions.
The implementation itself is based on a new member function
`MemRefType::getMaxCollapsableTrailingDims` that return the maximum
number of trailing dimensions that can be collapsed - trivially all
dimensions for memrefs with identity layout, or by examining the memref
strides stopping at discontiguous or statically unknown strides.
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