[clang] Remove profile available check for fsplit-machine-functions.

Enforcing a profile available check in the driver does not work with
incremental LTO builds where the LTO backend invocation does not include
the profile flags. At this point the profiles have already been consumed
and the IR contains profile metadata. Instead we always pass through the
-fsplit-machine-functions flag on user request. The pass itself contains
a check to return early if no profile information is available.

Differential Revision: https://reviews.llvm.org/D87943