[MachineOutliner] Do not outline debug instructions

The debug location is removed from any outlined instruction. This
causes the MachineVerifier to crash on outlined DBG_VALUE
instructions.

Then, debug instructions are "invisible" to the outliner, that is, two
ranges of instructions from different functions are considered
identical if the only difference is debug instructions. Since a debug
instruction from one function is unlikely to provide sensible debug
information about all functions, sharing an outlined sequence, this
patch just removes debug instructions from the outlined functions.

Differential Revision: https://reviews.llvm.org/D89485