[GlobalISel] IRTranslator: Use RAIIMFObsDelInstaller (#102379)
Similar to #102156.
runOnMachineFunction() installs the Observer correctly manually.
finishPendingPhis() doesn't install into the MF, but this currently
can't cause issues because DILocationVerifier doesn't care about changed
instructions.
Switch to RAIIMFObsDelInstaller in both places for consistency.