[LiveInterval] Add RemoveDeadValNo argument to removeSegment(iterator)

Add an optional bool RemoveDeadValNo argument to the
removeSegment(iterator) overload, for consistency with the other
overloads. This gives clients a way to remove dead valnos while also
getting an updated iterator returned (in the manner of vector::erase).

Use this to clean up some inefficient code in
LiveIntervals::repairOldRegInRange. NFC.

Differential Revision: https://reviews.llvm.org/D110560