[DebugInfo] Add replaceArg function to simplify DBG_VALUE_LIST expressions

The LiveDebugValues and LiveDebugVariables implementations for handling
DBG_VALUE_LIST instructions can be simplified significantly if they do not have
to deal with any duplicated operands, such as a DBG_VALUE_LIST that uses the
same register multiple times in its expression. This patch adds a function,
replaceArg, that can be used to simplify a DIExpression in the case of
duplicated operands.

Differential Revision: https://reviews.llvm.org/D83896