[memcpyopt] Teach memcpyopt to optimize across basic blocks

This teaches memcpyopt to make a non-local memdep query when a local query
indicates that the dependency is non-local. This notably allows it to
eliminate many more llvm.memcpy calls in common Rust code, often by 20-30%.

Fixes PR28958.

Differential Revision: https://reviews.llvm.org/D38374

llvm-svn: 319482