[FunctionComparator] Differentiate instructions passing different MDStrings (#69543)

Prior to this patch, differing metadata operands to two otherwise
identical instructions was not enough to consider the instructions
different in the eyes of the function comparator. This breaks LLVM
virtual function elimination, among other features.

In this patch, we handle the case where two associated operands are
MDStrings of different value. This patch does not differentiate more
complex metadata operands.

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Co-authored-by: Nuri Amari <nuriamari@fb.com>