Follow-up to #192289. Swap the remaining `std::unordered_set`/
`std::unordered_map` containers in `Instrumentation.cpp` for `DenseSet`/
`DenseMap`: the `BBToSkip` param and `Visited` local in
`hasAArch64ExclusiveMemop`, and `BBToSkip`, `BBToID`, `VisitedSet` in
`instrumentFunction`. Drop the now-unused `<unordered_set>` include.
The swap removes per-element heap allocations on the hot path, stops
inserting empty buckets on probes where a miss is possible, and replaces
hashed-bucket traversal over node-based storage with lookups over inline
`DenseMap` storage. `BBToID` reads keep `operator[]` since the map is
pre-populated for every basic block of the function, so no
default-construct path is ever taken. NFC.
Measured on `llvm-bolt -instrument` against a relocations-linked
clang-23: -1.3% instrumentation-pass wall time, peak RSS unchanged
(dominated by instrumentation output size).
2c56a63b49