This patch adds a command line flag for the machine function splitter
(added in rG94faadaca4e1). The command line reference has been updated
with the new flag and tests added to check the pass is run (requires
profile data) and that the driver has registered the correct option. A
warning is emitted if the option is enabled when no profile is provided.
Split machine functions using profile information (x86-elf only). On
other targets an error is emitted. If profile information is not
provided a warning is emitted notifying the user of backend