[MergeFuncs] Compare load instruction metadata

MergeFuncs currently merges load instructions with differing
semantically-relevant metadata, e.g. a load that has !nonnull
with one that does not.

Update FunctionComparator to make sure that metadata of both
loads is the same. Alternatively, it would be possilbe to ignore
the metadata during comparison, and then drop it during merging.

Differential Revision: https://reviews.llvm.org/D144682