Metadata extractor must tolerate missing accumulator function.

Looking up a general reduction accumulator function by name can fail,
just as looking up a foreach kernel function by name can fail: One of
the uses of MetadataExtractor is as part of the RSEmbedInfoPass, which
bcc_compat runs sufficiently late in the phase order that
RSKernelExpandPass has already run and the original (UNexpanded)
accumulator function may have been deleted as having no references (if
it has been inlined into the expanded accumulator function and is
otherwise unreferenced).

Bug: 23535724
Change-Id: I41b1fc9cc5c780174ddf9b85af71cb3381928bcf