[NFC][AsmPrinter] Refactor FrameIndexExprs as a std::set (#66433)

This avoids the need for a mutable member to implement deferred sorting,
and some bespoke code to maintain a SmallVector as a set.

The performance impact seems to be negligible in some small tests, and
so seems acceptable to greatly simplify the code.

An old FIXME and accompanying workaround are dropped. It is ostensibly
dead-code within the current codebase.