Fix PR4336: Iterating over use-def chains doesn't seem to be deterministic.

The problem was that BitcodeReader::materializeModule would read functions
from the bc file in densemap pointer key order (doubly non-deterministic!),
which would cause the use-def chains to be set up for globals in
non-determinstic order.  Non-determinstic use/def chains can cause 
nondeterminism in many places down-stream.

Many thanks to Julien Lerouge for putting together the pass in the PR that
shows the issue!

llvm-svn: 73470