[mlir] translate memref.reshape with static shapes but dynamic dims

Prior to this patch, the lowering of memref.reshape operations to the
LLVM dialect failed if the shape argument had a static shape with
dynamic dimensions.  This patch adds the necessary support so that when
the shape argument has dynamic values, the lowering probes the dimension
at runtime to set the size in the `MemRefDescriptor` type.  This patch
also computes the stride for dynamic dimensions by deriving it from the
sizes of the inner dimensions.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D126604