[DeadStoreElimination] Use SmallSetVector (NFC) (#79410)

The use of SmallSetVector saves 0.58% of heap allocations during the
compilation of a large preprocessed file, namely X86ISelLowering.cpp,
for the X86 target.  During the experiment, the final size of ToCheck
was 8 or less 88% of the time.

[DSE] Fix non-determinism due to address reuse (#84943)

The malloc->calloc fold creates a new MemoryAccess, which may end of at
the same address as a previously deleted access inside SkipStores.

To the most part, this is not a problem, because SkipStores is normally
only used together with MemDefs. Neither the old malloc access nor the
new calloc access will be part of MemDefs, so there is no problem here.

However, SkipStores is also used in one more place: In the main DSE
loop, ToCheck entries are checked against it. Fix this by not using
SkipStores here, and instead using a separate set to track deletions
inside this loop. This way it is not affected by the calloc optimization
that happens outside it.

This is all pretty ugly, but I haven't found another good way to fix it.
Suggestions welcome.

No test case as I don't have a reliable DSE-only test-case for this.

Fixes https://github.com/llvm/llvm-project/issues/84458.