[dfsan] Fix Len argument type in call to __dfsan_mem_transfer_callback

This patch is supposed to solve: https://bugs.llvm.org/show_bug.cgi?id=50075

The function `__dfsan_mem_transfer_callback` takes a `Len` argument of type `i64`; however, when processing a `MemTransferInst` such as `llvm.memcpy.p0i8.p0i8.i32`, the `len` argument has type `i32`. In order to make the type of `len` compatible with the one of the callback argument, this change zero-extends it when necessary.

Reviewed By: stephan.yichao.zhao, gbalats

Differential Revision: https://reviews.llvm.org/D101048