Revert "[memcpyopt] Teach memcpyopt to optimize across basic blocks"

This reverts r321138. It seems there are still underlying issues with
memdep. PR35519 seems to still be present if debug info is enabled. We
end up losing a memcpy. Somehow during store to memset merging, we
insert the memset after the memcpy or fail to update the memdep analysis
to account for the newly inserted memset of a pair.

Reduced test case:

  #include <assert.h>
  #include <stdio.h>
  #include <string>
  #include <utility>
  #include <vector>

  void do_push_back(
      std::vector<std::pair<std::string, std::vector<std::string>>>* crls) {
    crls->push_back(std::make_pair(std::string(), std::vector<std::string>()));
  }

  int __attribute__((optnone)) main() {
    // Put some data in the vector and then remove it so we take the push_back
    // fast path.
    std::vector<std::pair<std::string, std::vector<std::string>>> crl_set;
    crl_set.push_back({"asdf", {}});
    crl_set.pop_back();
    printf("first word in vector storage: %p\n", *(void**)crl_set.data());

    // Do the push_back which may fail to initialize the data.
    do_push_back(&crl_set);
    auto* first = &crl_set.back().first;
    printf("first word in vector storage (should be zero): %p\n",
           *(void**)crl_set.data());
    assert(first->empty());
    puts("ok");
  }

Compile with libc++, enable optimizations, and enable debug info:
$ clang++ -stdlib=libc++ -g -O2 t.cpp -o t.exe -Wl,-rpath=llvm/build/lib

This program will assert with this change.

llvm-svn: 321510