[MachineOutliner][NFC] Remember when you map something illegal across MBBs

Instruction mapping in the outliner uses "illegal numbers" to signify that
something can't ever be part of an outlining candidate. This means that the
number is unique and can't be part of any repeated substring.

Because each of these is unique, we can use a single unique number to represent
a range of things we can't outline.

The outliner tries to leverage this using a flag which is set in an MBB when
the previous instruction we tried to map was "illegal". This patch improves
that logic to work across MBBs. As a bonus, this also simplifies the mapping
logic somewhat.

This also updates the machine-outliner-remarks test, which was impacted by the
order of Candidates on an OutlinedFunction changing. This order isn't
guaranteed, so I added a FIXME to fix that in a follow-up. The order of
Candidates on an OutlinedFunction isn't important, so this still is NFC.

llvm-svn: 345906