[mlir] Fix bitwidth of memref-to-llvm constant.

One constant generated in MemRefToLLVM had a hardcoded bitwidth of
64 bits. The fix uses the typeConverter to create a constant that
matches the bitwidth of the provided by the data layout. The issue was
detected in an attempt to add a verifier to the LLVM ICmp operation that
checks that the types of the compared arguments match.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D135775