[mlir][memref] Add realloc op.

Add memref.realloc and canonicalization of the op. Add conversion patterns for
lowering the op to LLVM using unaligned alloc or aligned alloc based on the
conversion option.

Add filecheck tests for parsing and converting the op. Add an integration test.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D133424